fly on the wall idiom
暗中偷听者；暗中观察者
If you say that you would like to be a fly on the wall on an occasion, you mean that you would like to hear what will be said or see what will happen while not being noticed.
I'd love to be a fly on the wall when those two get home! 我想暗中听听那俩人回到家说些什么！
See also: fly-on-the-wall