a fly on the wall = an unnoticed observer
Let’s talk about the phrase a “fly on the wall.” If you are a fly on the wall, you hear something without other people noticing. Another way of describing it is a person who hears something private. This idiom is most often used to talk about something you wish you heard.
Megan: What are they talking about in that meeting?
Bob: They’re writing the new contracts.
Megan: I wish I could be a fly on the wall in there.
Bob: Me too, I want to know what they’re saying.
Talk about it: Tell a coworker or classmate about a time you wanted to be a fly on the wall!