A fly on the wall.

Usually, a fly is so small that it can enter a room unnoticed. It can sit on a wall and observe everything. 

In this cartoon, we see two lovers kissing on a bed. Another guy has changed into a fly, or has he? Maybe he is only wearing a fly costume hoping to fool the lovers so he can watch and listen to them. 

We usually wish to be 'a fly on the wall' and go unnoticed when we want to be somewhere secretly and see and hear what happens. 

Let's look at an example of this idiom in a conversation: 

Why's Maria so angry?
She found out about Paul's other girlfriend!
Does Paul know that she knows?
No. He isn't home yet.
What time does he usually get home?
Round about now. 
I'd love to be a fly on the wall when she talks to him.
Me too!