A fly on the wall. Have you heard that expression, "I'd love to be a fly on the wall when....." ? Basically, it means that you would like to be somewhere when something happens, but you wouldn't want people to notice you.
So, when your new girlfriend/boyfriend tells his/her best friend about your date last night, you'd love to be a fly on the wall to find out whether or not you were a real hit.
Or when your parents are deciding whether or not you should be allowed to do something, wouldn't it be great to be a fly on the wall? Then, you'd know what their objections are and you could try to convince them otherwise, without their knowing you knew.
Or how about being a fly on the wall when your teacher is making up a test? Or when the president is deciding whether or not to invade another country? Or when Bill Gates comes up with another idea?
Yeah, being a fly on the wall would let you in on all sorts of secrets.
Like what your parents are planning on buying you for Christmas -- and you think "No, not THAT stupid game. That's the one I liked LAST week." Then you'd know to drop hints about the new one you really really want.
I'll be a fly, thank you very much.