when the cats away, the mouse will play

When the cats away, the mouse will play is an idiom that is referring to the fact that when no one who is in authority is present, the subordinates will do as they please. It is referencing a pet cat and mouse in someones home, where when the cat is gone, the mouse feels free to come out from their hiding and roam the house. The idiom is used most often in situations like work where there is a boss and subordinates.