While the cat’s away, the mice will play.
When the cat’s away, the mice will play.

People will naturally take advantage of the absence of someone in authority to do as they like:

‘His parents are away for the weekend.’ ‘I see—while the cat’s away.’

However, it rings true that when the cat’s away, the mice will play,’ said Dronkers.

He left last night, straight from work, and as you know, while the cat’s away, the mice will play.

It’s certainly a case of while the cat’s away, the mice will play – what they get up to is barely legal!