As a former resident and frequent traveler to Hong Kong, R had never considered himself a political threat under the city's national security law, which ushered in an ongoing crackdown on dissent when it was imposed three years ago.
Until he was ushered into a small, windowless room at Hong Kong International Airport at the beginning of this year on arrival from his native Taiwan, and subjected to questioning and a full body search by both uniformed and plainclothes police.
"Who do you know in Hong Kong?" his interrogators wanted to know. "Who have you worked for, either salaried or unpaid, during your time in Hong Kong?"
A sign on the wall seemed reassuring: "You have the right to contact friends and relatives, and you have the right to a lawyer, as long as it doesn't obstruct official duties."
But R, who was initially detained by immigration officers, wasn't allowed to use his phone.
Later, a couple of plainclothes officers showed up and took his personal documents, including his passport, Taiwanese ID card and even his staff lanyard from his Taiwanese university, away for photocopying.
R, who asked not to be named for fear of further reprisals, was denied entry to Hong Kong, then loaded into a prison van with bars on the windows, for deportation back to Taiwan.
"That was the first time in my life I'd been inside a prison van," he said. "There were nine of them in there with me, watching me closely."
"They treated me like a terrorist."
Denying entry
R once worked in a Hong Kong university, yet his research doesn't directly touch on the city or its politics, he said. He has traveled there frequently in recent years, however, regarding it as a kind of second home.
While he did go and observe some of the mass marches of the 2019 protest movement, he was never a direct participant.
Occasionally, he would repost news about political developments in the city on Facebook.
R was officially denied entry to Hong Kong because he had "failed to meet the qualifications following comprehensive assessment." He won't be trying again for a few years.
The Hong Kong authorities have been denying entry to prominent democracy activists and rights campaigners for years, starting during the 2014 pro-democracy Umbrella Movement.
R still can't figure out what he has done wrong, given that his research doesn't even involve Hong Kong.
The Taiwanese government recently updated its travel advice for the city, warning its nationals not to run afoul of the national security law.
Possible infractions include wearing the wrong kind of T-shirt, or singing the wrong song in public, or otherwise displaying slogans relating to the protest movement.
Cracking down
The national security law – imposed by Beijing on Hong Kong from July 1, 2020 – ushered in a citywide crackdown on public dissent and criticism of the authorities that has seen senior journalists, pro-democracy media magnate Jimmy Lai and 47 former lawmakers and democracy activists charged with offenses from "collusion with a foreign power" to "subversion."
It applies to speech and acts committed anywhere in the world, and has been used to issue the leaders of a London-based rights group with a takedown order for its website.
Shouting or displaying protest slogans in a public place, including the banned "Free Hong Kong! Revolution Now!" or playing the British national anthem in public were also on the list of actions to avoid published by Taiwan's Mainland Affairs Council, an executive body in charge of the island's complicated relationship with China.
But the Council declined to comment on R's case when contacted by The Reporter recently, saying that it hadn't received any report of his treatment.
So who decides who gets onto the "blacklist" of people arriving in Hong Kong, and according to what criteria.
From around 2018
It's a murky area that Taiwan's officials were unable to shine a light on, aside from saying off the record that things started to get more risky for the island's nationals in Hong Kong in around 2018.
They have long been fair game for police in mainland China. Taiwanese political activist Lee Ming-cheh was detained and jailed for five years while on a trip there in 2017, with fellow activist Lee Meng-chu meeting a similar fate in 2019.
More recently, Chinese authorities arrested leading Taiwanese nationalist politician Yang Chih-yuan for 'secession' and detained Taiwan-based publisher Li Yanhe, known by his penname "Fucha," in Shanghai.
The recent charging of Yuen King-ting, 23, with sedition for posting banned Hong Kong protest slogans while she was studying in Japan has also heightened fears in Taiwan.
According to former Chinese University of Hong Kong anthropology professor Gordon Mathews, most foreign nationals are likely to be refused entry to Hong Kong if they are deemed suspect, unless they have been charged with a felony, rather than being detained and prosecuted.
But he doesn't expect there to be national security law-related issues for foreign nationals merely transiting at Hong Kong.
However, his advice specifically refers to people not born in China or Hong Kong, and who have never held citizenship in those territories. Foreign passport-holders who used to be Chinese citizens tend to have their foreign nationality disregarded, as in the case of Jimmy Lai, he said.
Arbitrary list?
It's not just academics from Taiwan who could be at risk. Foreign journalists working in Hong Kong could also be at risk -- it's just very hard to say how much, according to Reporters Without Borders' East Asia bureau chief Cédric Alviani.
That's because the Chinese government can always override its own guidelines and detain anyone arbitrarily.
Alviani also warned that journalists of Chinese descent holding foreign passports are particularly vulnerable to this kind of treatment, citing the cases of Cheng Lei, Yang Hengjun and Jimmy Lai.
Lin Thung-Hong, who directs the Center for Contemporary China Studies at Taiwan's National Tsinghua University, agreed that the “blacklist” can be an arbitrary thing.
"It's impossible and pretty meaningless to try to second-guess where Hong Kong's political red lines are drawn," Lin told The Reporter.
"An authoritarian government will never allow these lines to be clearly drawn, because [opacity] is the best way to spread fear and facilitate control," he said, adding that there are no rules of thumb for figuring out if you have been blacklisted or not.
But he said his university warns researchers traveling to both China and Hong Kong to go with empty phones and hard drives, and to use only custom-made alphanumeric passwords, not facial recognition or other identification factors.
If detained, stay calm, never admit to any crime, and don't mention anyone else's name or details, he said.
Translated by Luisetta Mudie. Edited by Malcolm Foster.
This is an edited version of a collaborative report by RFA's Mandarin Service and The Reporter , a Taiwan-based investigative magazine.