A North Korean defector in her 30s, whose surname was only given as Jung, changed her itinerary to China after receiving a text message from the police officer responsible for protecting her.
The text message warned: "Recently, North Korean defectors have continued to be refused entry, deported, arrested or detained by the Chinese authorities when entering or transiting through China."
The text message also advised recipients to "avoid traveling to China or choose other destinations" and asked those who have already traveled to China to "provide itinerary and flight information to the police responsible for protecting themselves."
Ms Zheng told the BBC: "After receiving the text message, I canceled my trip to China and went to Japan instead."
The BBC found that many North Korean defectors under police protection in South Korea had received similar travel warnings to China.
As of the end of August this year, a total of 20,401 North Korean defectors in South Korea were under police protection, accounting for 64.7% of all North Korean defectors living in South Korea.
After North Korean defectors register their address in South Korea, they are generally provided with protection for up to five years, but this can be extended depending on their wishes and safety needs.
So, why do these North Korean defectors who now hold South Korean nationality and passports still receive warnings to travel to China?
Why are travel warnings issued?
A police warning text message obtained by the BBC stated that entry and exit inspections have been tightened after China recently implemented a temporary visa-free policy.
The text message also stated that many North Korean defectors who encountered problems when entering and exiting China were found to have used forged passports or held Chinese household registration documents during their stay in China.
Recipients are reminded that if they are arrested, detained or encounter other difficulties, they should immediately contact the police responsible for their protection and request consular assistance from the South Korean government.
This summer, Seo Jae-pyung, president of the North Korean Defectors Association, also received a call from the police responsible for protecting him, asking if he planned to travel to China and reminding him to be extra careful once he went to China.
The warning quickly spread among defector communities. Xu Zaiping said that group chat rooms with dozens of North Korean defectors have been actively sharing police warnings, personal experiences in China, and information about China's entry and exit procedures.
A local police protection officer who had sent text messages to North Korean defectors told the BBC that the warning was issued in accordance with instructions from the South Korean National Police Agency to "ensure the safety of North Korean defectors within their jurisdiction."
He added that the text messages also included information on international agreements stating that in the event of arrest or detention, the person concerned has the right to receive consular assistance.
An official from the South Korean National Police Agency said that the relevant guidelines were issued after the South Korean Ministry of Foreign Affairs requested cooperation.
The official said that in view of the continuous problems encountered by North Korean defectors at Chinese airports, the Ministry of Foreign Affairs asked the police to explain to them the risks of traveling to China and their rights to obtain consular assistance.
However, the National Police Agency stated that it is not clear how many North Korean defectors have been refused entry, arrested or detained in China recently, because North Korean defectors are not obliged to declare to protective police officers when they go abroad.
"I was asked an unexpected question when I entered the country"
Before the government issued the warning, some people who fled from North Korea to South Korea and obtained South Korean citizenship said they were subject to extra checks when entering China.
Kim So-yeon, who is engaged in singing in South Korea, is one of them.
Kim So-yeon recalled that last fall she went to Qingdao, China for interpretation work, and encountered an unexpected cross-examination during immigration inspection.
She said that after pressing her fingerprints, the immigration officer spent a long time comparing her appearance with the information on the screen, and then asked her several questions.
"The officer suddenly asked me if I had ever lived in China before and if I had ever had a Chinese household registration."
After Kim So-yeon escaped from North Korea, she lived in China and did hold a Chinese household registration document at the time. However, she denied the incident out of fear of possible consequences.
When she answered in Chinese, the other person asked her how to learn Chinese. Kim So-yeon replied that she learned it in South Korea, but the cross-examination did not stop.
According to her account, the other party then gave her a piece of paper and a pen and asked her to write Chinese characters by hand. She felt the other party was trying to compare her handwriting with old records.
"The person asked me to write a Chinese character that I recognized, so I deliberately used an unusual stroke order."
After another round of questioning and luggage inspection, she was finally allowed to enter China.
Kim So-yeon said she suspected that Chinese authorities already knew she had lived and registered there in the past because staff asked relevant questions immediately after collecting fingerprints.
She has not traveled to China since that trip.
How does China identify North Korean defectors?
So how can Chinese authorities identify people entering the country on South Korean passports who have been North Korean defectors in the past and have ties to China?
In the past, the Korean resident registration number itself has sometimes provided clues.
Before 2007, many North Korean defectors settled in South Korea after passing through the "Asianawon", a resettlement agency for North Korean defectors operated by the South Korean government in Anseong, Gyeonggi Province. The resident registration numbers they were issued were based on the location of the Asiana Institute.
Therefore, among the seven digits after the hyphen, the first three digits are "125" for men and "225" for women.
According to reports, this means that North Korean defectors may be identified when applying for visas or going through immigration procedures, and some ordinary South Koreans who use the same area code are sometimes mistaken for North Korean defectors.
The Korean government revised the system in 2007, and the resident registration number was changed to be assigned based on the person's actual place of residence, rather than the location of the Asiana Institute.
In 2009, the relevant laws were revised again, allowing North Korean defectors who had previously been issued a registration number related to the Asiana Institute to apply for a new number.
In addition to South Korean resident registration numbers, local identity records used by North Korean defectors when they lived in China may also be clues to their identities.
Xu Zaiping explained that some North Korean defectors who had lived in China for a long time had spent money to obtain and use Chinese household registration or identity documents in other people's names for their own safety.
He said that he knew of some cases where North Korean defectors returned to China after settling in South Korea and were asked to deal with identity record issues left in the past.
Biometric information, such as fingerprints and facial data, may also play a role.
A research report published by the North Korean Human Rights Information Center (NKDB) last year stated that biometric information such as fingerprints and facial recognition information had been collected from some North Korean defectors living in China.
Shin Dong-hwi, who participated in the study, told the BBC: "The basic information collected includes fingerprints and facial information. In some areas, authorities also collect blood samples or record voices."
However, Shin Donghui emphasized that these measures are not specifically targeted at North Korean defectors.
"This appears to be part of a broader policy targeting foreigners living illegally in China. Since China does not recognize North Korean defectors as refugees, these policies will also apply to them."
He analyzed that if North Korean defectors used other identities or registration information in China before moving to South Korea, their current identities may be linked to past records when they re-enter China in the future.
The BBC spoke to a North Korean defector who had been stopped trying to enter China in the past.
Lee Han-sol (pseudonym and transliteration, Lee Han-sol), who is in his 50s, tried to enter China many times after settling in South Korea, but was refused entry three times around 2018. She said that she had traveled to and from China many times before and had never encountered any problems.
What puzzled her most was that the Chinese authorities seemed to know her identity before taking her fingerprints.
"I was waiting in line to enter the country, and before my fingerprints were taken, three police officers came over. They looked at my passport and said, 'You are a North Korean defector, right?' I kept answering that I am a citizen of the Republic of Korea."
Lee Hansol has lived in China in the past and used a Chinese ID card. She was also detained there in 2009 and knew her fingerprints might still be in official records.
But because she was stopped before any fingerprint scans were taken, it's still unclear how authorities identified her.
Lee Han Sol said that Chinese officials only told her that she could not enter the country because she was a North Korean defector, but did not explain the specific reasons or how to verify her identity.
She said that each time she was taken to a separate area and detained for about 24 hours, and her mobile phone and passport were confiscated until she boarded a flight back to South Korea.
Unable to enter China, she ultimately had no choice but to return home and has never returned to China since then.
With visa exemption, worries are not eliminated
South Korean passport holders can currently enter China visa-free and stay up to 30 days for tourism, business, family visits, exchanges or transit purposes.
According to information from the Chinese Ministry of Foreign Affairs, the relevant policy will be implemented until December 31, 2026.
Still, many North Korean defectors are reluctant to travel to China.
Ms. Zheng, who canceled her trip to China, said she was worried that the Chinese authorities might share the information of North Korean defectors with North Korea and that North Korean defectors might be kidnapped in China.
Xu Zaiping also said: "Many North Korean defectors feel that the risk is too high. They believe that there is no reason to go to a place that may put themselves in danger."
The BBC asked the South Korean government how many North Korean defectors who have obtained South Korean citizenship have been refused entry, arrested or detained in China recently.
South Korea's Ministry of Foreign Affairs responded to the BBC: "We count incidents involving Korean citizens overseas by country and region, but we will not make separate statistics based on specific identities, such as whether they are North Korean defectors."
The Ministry of Foreign Affairs added that it has been cooperating with the National Police Agency and other relevant agencies to ensure the safety of Korean citizens overseas.
However, the Ministry of Foreign Affairs declined to disclose details of the discussions that led to the recent warning.
The Ministry of Foreign Affairs also pointed out that according to the consular agreement between South Korea and China, once the Chinese authorities arrest or detain a South Korean citizen, they must notify South Korean consular officials within four days regardless of whether the person concerned actively requests notification.
When asked whether there had been any confirmed cases in the past of North Korean defectors who had obtained South Korean nationality being repatriated to North Korea or handed over to North Korean authorities by China, the Ministry of Foreign Affairs said: "According to the information available to the Ministry of Foreign Affairs, there have been no cases of North Korean defectors who had obtained South Korean nationality being repatriated to North Korea or handed over to North Korean authorities."
The BBC also submitted written questions to the Chinese Ministry of Foreign Affairs to inquire whether South Korean citizens who were former North Korean defectors face different entry inspection standards than other South Korean passport holders; how many people have been refused entry, questioned or detained recently; if there are relevant cases, what are the reasons?
