While the cat's away, the mice will play.

"While the cat's away, the mice will play" is an international proverb. It's well known in Spanish and in many other languages too.

It basically means without supervision, people will do as they please. i.e. When a person in authority is away, those under the person’s rule will enjoy their freedom.^[1]

People whose behaviour is strictly controlled go over the top when the authority is not around, which is why most teenagers have parties when their parents have gone on holiday. The parents are the scary authority figures, but the cat's away and the kids are the mice partying and enjoying their freedom.^[2]

The Spanish version is "Cuando el gato duerme, los ratones hacen fiesta" and/or "Cuando el gato duerme, los ratones se divierten".

¹ The New Dictionary of Cultural Literacy, Third Edition. 2002.
² UsingEnglish.com