The bail system only works if suspects repay the trust placed in them. There is now a concern the police will be less sympathetic to others, a case of once bitten, twice shy.

But Chow’s case raises broader questions. She was arrested on suspicion of breaching the national security law by colluding with “foreign elements” in August 2020.

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The activist was jailed in December that year for her conduct during civil unrest in 2019, but released in June 2021. She has, therefore, been on police bail for an extraordinarily long time.

After more than three years, you would expect either charges to be brought or the suspect released unconditionally. Why has the investigation dragged on so long without a decision being made?

Bail is, no doubt, preferable to being held in custody. But it is a stressful experience, with conditions usually including a requirement to report regularly to the police. Suspects live in fear of their bail suddenly being revoked or conditions tightened. Their future is uncertain.

Chow said the experience caused her mental health to deteriorate, and is suffering from anxiety and depression.

The police had good reason to allow Chow to study in Canada, allowing her a semblance of normal life while the long investigation continued. Now they feel she duped them.

But the conditions under which Chow says her passport was returned to her have raised eyebrows.

The police are entitled, under rules relating to the national security law, to require a suspect to surrender their travel document. But the arrested person can apply for it to be returned if they have a good reason for travelling outside Hong Kong. Applications can be granted if the police believe a refusal would cause unreasonable hardship. They are expected to take all circumstances into account, including the interests of the investigation.

Chow says she was required to show evidence that she had a place on a course overseas. But that was not all. The activist claims police took her on a day trip to Shenzhen to attend an exhibition on China’s economic reforms and to visit the headquarters of tech giant Tencent. She even had to pose for photographs.

The activist says she was also compelled to write a letter of thanks to the police for arranging the trip to help her “learn about the accomplishments of the motherland”. She had to write a letter regretting her past political activities, too.

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The authorities have not confirmed or denied this happened. Such requirements go well beyond what would normally be expected of a suspect on bail.

An explanation is needed. Were such conditions imposed? If so, were they part of her bail arrangements? Which powers were the police exercising? Have similar steps been taken with other suspects?

The police might argue that in assessing whether Chow was a flight risk, they wanted to ensure she had mended her ways. The patriotic trip and writing of repentance letters might, at a stretch, be seen in that light.

Bail conditions should be confined to ensuring suspects are available and stay out of trouble. They are not tools for rehabilitation or re-education. After all, Chow has not even been charged with a national security offence, let alone convicted.

Clarity is required. The measures alleged by Chow are unfamiliar in Hong Kong and more in keeping with those adopted in mainland China. The city, with its very different legal system, should not allow the lines to be blurred.