Fly on the wall
Alludes to the position of being able to freely observe a situation without being oneself noticed.
This is an American phrase that originated there in the 1920s. The first citation of it that I can find is from The Oakland Tribune, February 1921:
"I'd just love to be a fly on the wall when the Right Man comes along."
It is now most often used in relation to 'fly on the wall documentaries', which are films of real life situations supposedly made without affecting the behaviour of the participants. It is de rigeur for participants in such films to comment along the lines of 'We just got used to the camera crew and after a while we just ignored them'. Well, maybe. I must say that if I had half a dozen strangers trooping around my house I think I would know that they were there.