A fly on the wall
Definition 1 of 1
Pro Tip 1/3Invisibility Wish
Used when someone wants to overhear others without being noticed, often in private or sensitive situations. 
If I could be a fly on the wall during their secret meeting, I’d learn all their plans.
Pro Tip 2/3Curiosity Context
Expresses a strong curiosity about what will be said or done in one’s absence, especially in intriguing scenarios. 
Imagine being a fly on the wall at the celebrity party - the stories you'd hear!
Pro Tip 3/3Not Literal
It's a metaphorical expression and doesn't mean someone wants to be an actual insect or to endorse eavesdropping. 
I don't really want to be a bug, but I'd be a fly on the wall to hear their true opinions.