It's all in the name

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.

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.
- Apartment buildings are built with thin walls so you are not safe even within your own home.
  Pshaw. Next you'll be claiming that our phones are built to listen to our conversations.
Yeah, plains clothes officer in the US. You're repeating the meme.
Sooooo, the NSA, the CIA, the FBI, the DEA...
Take your pick.
e.g. secret police can arrest you on whatever charges (don’t have to be revealed)
1984 Bail Reform Act
2001 PATRIOT Act
- Literally 1984
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
Like the CIA blacksites, Gitmo, the Fed vans that pulled and kidnapped protestors during the BLM protests? Or the fact that police can get away with murder in broad daylight and people who fight it are labeled terrorists who hate the country?
Can't we just enjoy our anti-American propaganda in peace
Oh so the international FBI, what was it called? ah CIA right
If you're getting arrested by secret police, then you fight to the death. Otherwise, you'll spend the rest of your life wishing you did.
- Typically they have the guns and you don't, or if you do they'll kill you then disappear you rather than the other way around
Literally 1984

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.

- what’s the deal with the white arm bands?
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.