A secret police is something different from officers without uniforms. A secret police is an instrument of the ruling party to oppress opposition and are part of a "justice" system outside of the official one, e.g. secret police can arrest you on whatever charges (don't have to be revealed) and you'll be put into a secret prison where your relatives can't find you. The fact that they don't wear uniforms goes beyond why regular police goes it: it's to create an atmosphere of fear that everyone around you could be secret police, and that they can just arrest you, and there's nothing you can do.
Yeah their distinction is completely arbitrary and meaningless. Of course the widespread knowledge that anyone you could interact with could be a plainclothes officer or a federal agent instills a certain amount of paranoia in Americans, the difference is that just we have massive and effective propaganda machines convincing us that America secret police are all good guys while also telling us that non-Western secret police are mustache twirling villains.
Yes. Secret Police makes people never relax. You never know who's watching, you never know who's listening. Your neighbours and teachers could be informants too. Apartment buildings are built with thin walls so you are not safe even within your own home. It's a constant source of terror.
It's also why people older than me don't talk to strangers, and especially, to foreigners.
Last couple of evenings has been cool enough to not have window units running. The neighbor's drunken bellowing has been off the chain. Yes, I heard it over the AC but without is truly revealing, and impossible to sleep.
Plain clothes are always incredibly obvious and oblivious to that fact. I've known them to stop and search people because they are baffled someone spotted them.
IME they try too hard to look like street people, but it doesn't work because their clothes are clean and they bathed, your average skel substance abuser hasn’t seen a shower in 24+hrs even if they got new donated clothes.