A Hong Kong man has allegedly disappeared while returning to Hong Kong from Shenzhen, China.

"Pray for me"

The 28-year-old man, Simon Cheng, is a trade and investment officer at the British Consulate General in Hong Kong.

He had travelled to Shenzhen in the morning for a business trip.

According to a statement put out by the man's family on Facebook on Wednesday, Aug. 21, they lost contact with him on Aug. 8 night just as he was about to go through the immigration customs between Shenzhen and Hong Kong.

His girlfriend, a Taiwanese with the surname Li, had allegedly received WeChat messages from him at about 10.40pm saying he was going to pass through customs.

He had also written, "pray for me".

That was the last time his family heard from him.

Detained at West Kowloon station

According to Financial Times, Li said Cheng was at the West Kowloon station, which allows travellers to go between Hong Kong and Shenzhen via high-speed rail.

The station's immigration checkpoint area is the first piece of land in Hong Kong to be designated legally as mainland Chinese territory.

Reports have emerged of travellers going into mainland China having their phones searched for videos and photos related to the anti-extradition bill protests, and even have their private messages with friends and family scanned.

"Administratively detained"

According to his family's statement, they sought assistance from the Hong Kong police and the Immigration Department on Aug. 9, and were told to go to the Ministry of Public Security in mainland China to report a missing case.

The next day, his family were told by Hong Kong's Immigration Department, which had contacted the Hong Kong Economic and Trade Office in Guangdong province, that Cheng had been "administratively detained".

"Administrative detention" allows individuals to be held without being charged.

The British embassy in Beijing confirmed that Cheng had been detained, FT reported.

However, the family said in their statement they could not obtain further information on why he was detained, where he was held, or how long he was to be detained.

Not found in local detention centres

The family's lawyer said the case is being handled by the Shenzhen municipal public security bureau, which is in charge of law enforcement and immigration control issues.

But the lawyer could not find Cheng at local detention centres, nor police stations in Shenzhen and Guangzhou.

The family also said they have yet to receive any documentation confirming that Cheng has been formally detained by the authorities, adding that a Notice of Administrative Detention was supposed to be sent out within 24 hours of a person's detention.

Detention believed to be related to participation in demonstrations

According to FT, Cheng is believed to have been detained due to his involvement in the anti-extradition bill protests.

However, Li denied that he took part in the demonstrations, saying there was "no reason" for him to be detained.

Not the only Hongkonger detained

Cheng was not the only one who was detained by mainland Chinese authorities at the immigration customs.

Several accounts of Hongkongers being detained while passing through customs were posted on social media.

Most were held for a few hours to a day.

In one particular case, an 18-year-old protester was detained at customs when he was found to have an "anti-extradition bill" sign in his bag.

He has since returned to Hong Kong safely.

In another alleged case of detention, a Hong Kong person was detained after being found to have a photo of the protests.Top image via Simon Cheng/Facebook