A Fly on the wall.

I have a friend that remembers hearing the phrase, "a fly on the wall", when he was a child. It was related to a verse in a fariy tale or rhyme. He was rasied in England. However he can't recall it's name.
I'm having very little luck with this. Help.
Thanks...

I can't help with the origin, but the expression is very common in the UK - there's even a TV documentary style named after it. It means very unobtrusive observation of situations. Surely also so in the US?