a fly on the wall

Từ điển Oxford Advanced Learner 8th

a fly on the ˈwall idiom
a person who watches others without being noticed
•I'd love to be a fly on the wall when he tells her the news.
•fly-on-the-wall documentaries (= in which people are filmed going about their normal lives as if the camera were not there)
Main entry: ↑fly_idiom