The bird has flown
Definition 1 of 2
Pro Tip 1/3Escape Context
Used when someone has avoided capture or an undesirable situation. 
The spies realized the bird has flown from the safe house.
Pro Tip 2/3Unexpected Absence
Implies sudden or unannounced departure of a person or entity. 
He wasn't in his office; the bird has flown.
Pro Tip 3/3Evasive Subject
Can indicate a person has actively evaded others or gone into hiding. 
When the reporters arrived, the celebrity had flown.