The British Foreign Office on Tuesday said it was “extremely concerned” by reports that trade and investment officer Simon Cheng Man-kit of the Hong Kong office has been detained by Communist China while traveling to the mainland for a business event. Cheng was listed as a “missing person” for over a week before there was any official confirmation he was in custody. The Chinese government has not explained why he was arrested.

Cheng, 28, disappeared 12 days ago while returning from a business trip to Shenzhen, the tech-hub mainland city closest to Hong Kong. Shenzhen is the city where China has been massing ominous police and paramilitary forces to signal Hong Kong protesters that Beijing’s patience is not without limits. The latest dark messaging from the Chinese Communist Party is that Shenzhen might replace Hong Kong as China’s business gateway to the outside world if Hong Kong proves itself to be ungovernable.

Cheng planned to return home to Hong Kong by train immediately after attending the conference in Shenzhen on August 8, as chronicled by the Hong Kong Free Press:

At 10:37pm on the day he left Hong Kong, Cheng told his Taiwanese girlfriend that he was on a high-speed rail train to return to Hong Kong, “ready to pass through the boarder [sic],” according to a screenshot. “Pray for me,” Cheng told his girlfriend in a message. The Express Rail Link has an arrangement in which border checks are not conducted in the mainland but at a joint checkpoint at the underground levels of the West Kowloon Terminus. Cheng could have been detained at the joint checkpoint, which is considered Chinese territory. Cheng did not return to work on August 9. According to his girlfriend, the British Consulate-General told Cheng’s family members to report the case to the police after they met with Nicola Barrett, a consul at the consulate. There were no arrests made at the mainland Chinese police station of the West Kowloon Terminus on August 8 and 9, an officer of the station told HK01 in a phone call. Cheng’s girlfriend said a unit at the Immigration Department, which provides assistance to Hong Kong residents outside of Hong Kong, told her on August 10 that Cheng was under administrative detention in mainland China, but the reason and location of his detention were unknown.

The West Kowloon Terminus of the Hong Kong rail station was one of the biggest controversies in the city before the attempt to pass a new extradition law cause the current protest movement to erupt. Part of the terminus was essentially classified as Chinese soil, governed by mainland law instead of Hong Kong’s special autonomous laws, for the stated purpose of making rail travel to Shenzhen easier. The idea was that travelers could pass through border security and “enter” China without leaving the rail station, rather than going through a cumbersome customs check when their train actually crossed over the Hong Kong border.

Critics said the terminus arrangement compromised Hong Kong’s autonomy and could make it easier for mainland police to arrest people from Hong Kong and drag them back to China. One reason protests against a proposed extradition bill broke out was that critics feared China would use easier extradition procedures to grab people from Hong Kong and “disappear” them into the opaque and politicized mainland judicial system.

The British Foreign Office said on Tuesday it was “seeking further information from authorities in Guangdong Province and Hong Kong” about Cheng’s whereabouts and condition.

“We are extremely concerned by reports that a member of our team has been detained returning to Hong Kong from Shenzhen,” the Foreign Office said.

Cheng works for the Scottish Development International department of the British consulate. The Scottish government is reportedly working with the Foreign Office on his case.

The South China Morning Post reported on Tuesday that a lawyer on the Chinese mainland hired by Cheng’s family “could not find any record of him in any detention center in Shenzhen or Guangzhou.” The family has been told only that Cheng is being held under “administrative detention” rules, which nominally require Chinese authorities to launch a formal criminal investigation no later than August 23.

“At first I didn’t want his case to go public because I was worried exposure would further jeopardise him, but now I really don’t know what else I can do,” Cheng’s girlfriend, identified only by her surname “Li,” told the SCMP.

Li said the family’s encounters with the authorities were frustrating, as Cheng’s employers seemed reluctant to get involved with the case and would not help the family hire mainland legal assistance because Cheng was supposedly “off-duty” when he disappeared. Hong Kong officials refused to tell the family which arm of the mainland Chinese government arrested him.

Li told the SCMP she and Cheng did not attend any Hong Kong protests together, but she allowed for the possibility he attended one without her, or might have interacted with the protest movement online.

The BBC quoted travelers who spoke of disturbing Chinese behavior on the Hong Kong border, including intrusive security checks and searches of personal electronics for material relating to the Hong Kong protest movement. One traveler from Hong Kong said Chinese police forced her to delete a hundred photos of the protests from her phone before she was allowed to cross the border.

“My observation is that they check two people out of three. The situation is quite severe. None of my friends want to go to mainland China now,” she said, a sentiment that might grow more intense if the disappearance of Simon Cheng Man-kit blows up into a major international incident.