A very good question.
In all these years, a lot of my friends - of all colour, origin, education, occupation , you name it - have been involved with the police one or the other way. Some, like the OP have had that misfortune to fall under the radar merely because of how they dress(ed) up. Like black , most recently. While black attire might have given an impression of a "protester '(hence " suspicious"), I, for one, have never been "stopped and questioned" for the same reason.
People used to say I'm an easy target of suspicion. When I was a teenager, I looked like someone who carried heroine around (what they said) HAHA- because, I dressed up that gothic way. You know how it is. My skin tone is somewhat fair, somewhat pale yellow - inherited from an Asian mother and a Caucasian father - that means I could fit into any race, and hence, prone to vulnerability at anytime .But well, I walked past the police chaps a thousand times in that "druggy person" look but wasn't stopped once. Sometimes, I'd walk (or do so even now) without the HKID card (intentionally or unintentionally). but have been lucky enough to go "unchecked ".
That leaves me with the question - what exactly could be those confounding " criteria " to look "suspicious " ?
(P.s- not bragging about me not being stopped and searched/questioned. I'm just, let's say, enthralled by the confusing law).