Hong Kong (CNN) A train station in the heart of Hong Kong could soon become a symbol of Hong Kongers' worst fears about China, after the detention of a UK consulate employee.

Simon Cheng, a Hong Kong citizen working for the British consulate, has not been heard from since August 8, when he told his girlfriend he had boarded a high-speed train, traveling from the Chinese city of Shenzhen to Hong Kong. "Pray for me," he texted her as he approached Chinese immigration, according to screenshots seen by CNN.

Beijing confirmed Wednesday that Cheng had been detained by police under a sweeping security law , but did not specify why -- the law cited for his detention applies to numerous broad offenses.

Friends fear his detention could have been linked to recent pro-democracy protests, noting that Cheng had shared pro-democracy images on social media and had voiced support for the city's ongoing protest movement. His detention comes amid reports Chinese immigration officials are regularly searching travelers' phones and bags as they cross the border for evidence they have taken part in protests.

Cheng's detention is even more symbolic for where it may have taken place: Not at the primary border crossings of Lok Ma Chau and Lo Wu, across from the Chinese city of Shenzhen, but in the heart of Hong Kong.

The high-speed train between Shenzhen and Hong Kong only passes through one immigration checkpoint: West Kowloon station, shared by both China and Hong Kong on the tip of the territory's northern peninsula. At a demonstration outside the British consulate Wednesday, protesters said Cheng's apparent arrest was likely one of many, and linked his detention to longstanding fears about the station's immigration arrangement.

Last year, a widely-criticized deal between Chinese and Hong Kong authorities placed part of West Kowloon station under Chinese law, allowing Chinese immigration officials and police to operate there. It was the first time such measures have been permitted in Hong Kong.

While Cheng could have been detained at any border crossing, an arrest at West Kowloon would be particularly worrisome to Hong Kongers who already feared that the station would be used by China to extend its control in the city. Reacting to Cheng's detention, Gary Fan, a pro-democratic lawmaker who voted against the plan, said "this case proves my nightmare came true."

'Thin end of the wedge'

Fan won't be the only person in Hong Kong who fear that Cheng's case confirms their suspicions about the shared station. The decision to effectively cede part of Hong Kong to China in order to allow Chinese officials to operate in West Kowloon station was roundly criticized by pro-democracy lawmakers and legal groups in the city before it came into effect late last year

The government defended the arrangement as necessary for the smooth operation of the $10.7 billion station, which linked Hong Kong to China's vast high-speed rail network. Officials said conducting immigration checks in Hong Kong -- the start or end point for all journeys at the station -- streamlined the process and avoided an additional check and delay at the Chinese border.

Hong Kong is governed under a policy known as "one country, two systems," which guarantees freedoms that are unavailable to those in the mainland, such as the right to protest, the right to a free press and freedom of speech.

Critics of Beijing worried the move set a dangerous precedent in allowing Chinese law to operate in Hong Kong, and would make it easier for officials to detain critics of Beijing and take them across the border.

The case of several Hong Kong booksellers , two of whom were allegedly abducted by Chinese agents from Hong Kong and Thailand respectively, hung large over the project.

"It's the thin end of the wedge," Hong Kong legal commentator Antony Dapiran told CNN last year . "(They have) effectively turned a piece of the station into a piece of mainland China."

Multiple protests were staged against the plan, and pro-democracy lawmakers tried unsuccessfully to block it in the city's parliament. The Hong Kong Bar Association warned the plan was "unconstitutional" and strongly criticized a decision by the Chinese parliament -- which is the ultimate court of appeal for constitutional matters in the city -- to approve it, saying that move "severely" undermined confidence in the rule of law.

The flags of Hong Kong (left) and China are seen hoisted outside the West Kowloon train station of the High Speed Rail Link to Guangzhou as sales counters were opened to the public for the first time in Hong Kong on September 10, 2018.

No place for Hong Kongers

After West Kowloon began operation in September last year , it appeared at first as if concerns around the plan were overblown. Criticism shifted instead to the huge cost of the station , after it became clear demand for the high-speed rail link was not as strong as initially suggested.

Then came the recent protests, sparked by a now-shelved extradition law with China, which some feared could be used to spirit critics of Beijing across the border to face punishment under a legal system notorious for human rights abuses and a near 100% conviction rate.

Opposition to the bill was rooted in the same fear as earlier opposition to the station's joint checkpoint arrangement -- that Beijing's control was slowly expanding, and increasing Hong Kongers' exposure to a system with a long record of abusing human rights.

As protests stretched into a summer of discontent, Chinese authorities across the border began ramping up police and military presence to guard against any spreading unrest. A buildup of paramilitary forces on the Shenzhen side of the border was highly publicized in Chinese state media and officials began conducting more stringent checks of travelers going in and out of Hong Kong.

One Hong Kong man, who recently crossed back from China at the Lok Ma Chau checkpoint, told CNN border police appeared to be "picking at random" people to be subjected to extra searches. He said he was asked whether he was a member of any protest groups and that police searched through his phone, looking at photos and other apps.

"I put my photos app on the last page of the phone, so it took him a while to find it. After that he kept scrolling checking literally all my photos," the man said. "I had very little visibility as to what the police officer was doing with my phone. He was standing facing me and holding my phone vertically facing me. All I (could see) was his gesture."

Unlike Cheng, he was released after around five minutes.

The detention of any Hong Konger as a result of the protests would attract outrage from the opposition movement -- newly emboldened by last weekend's huge turnout for a peaceful march. And the detention of a Hong Konger inside the city itself, under a plan that many warned would enable Chinese officials to go after opponents in the city, could open up a whole new front in the almost three-month long anti-government protests.