私服警官を英語でなんと言うか？

plainclothes (police) officer です。22:32 and plainclothes officials arrived and familiar questions were asked. 私服がやってきて、ありきたりの質問をしてきた。

Exposing The Changing Face Of Superpower China
https://www.youtube.com/watch?v=qLsURI9uFWk  0:05 the man here in a white T-shirt and jean shorts is Carlos Mercado. it's late August, 2013, in Brooklyn New York. and plainclothes officers are arresting him for selling two packets of heroin. 15 hours after entering the Rikers Island jail complex, he'll be dead.
Countdown to Death at Rikers Island | The New York Times
https://www.youtube.com/watch?v=sBmH3FH0yws  0:25 Hey, sheriff, almost didn't recognize you in plain clothes. well, t's my day off. 保安官、私服だとちょっとわかりませんね。非番なのさ。
The Last Stand Trailer

練習問題解答
a-6602 plain clothes
b-6442 plainclothes officers