Somebody's back is turned
Definition 1 of 1
Pro Tip 1/3Literal Meaning
The phrase can mean someone's back is physically turned away, not just metaphorically. 
He whispered the secret while his friend's back was turned.
Pro Tip 2/3Sneaky Action
Often used to describe taking a sneaky action without someone noticing. 
The cat stole the fish when the owner's back was turned.
Pro Tip 3/3Missed Observations
Implies someone missed seeing something important while not looking. 
The artist painted a mural when the city's back was turned.