1
general::
Without supervision, people will do as they please, especially in disregarding or breaking rules. For example, As soon as their parents left, the children invited all their friends over ? when the cat's away, you know. This expression has been a proverb since about 1600 and is so well known it is often shortened, as in the example.
American Heritage Idioms
2
general::
Phrase(s): When the cat’s away, the mice will play.
Prov. When no one in authority is present, the subordinates can do as they please. • When the teacher left for a few minutes, the children nearly wrecked the classroom. When the cat’s away, the mice will play. • Jill: You shouldn’t be reading a novel at your desk. Jane: But the boss isn’t here. And when the cat’s away, the mice will play.
McGrawhill's American Idioms And Phrasal Verbs