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

猫儿不在家，老鼠就玩耍；猫儿不在耗子闹。

Most likely. When the cat's away, the mice will play.

很有可能。猫儿不在，老鼠成精。

Original proverb: When the cat's away, the mice will play.

英语谚语：猫儿若不在，老鼠闹翻天。

When the cat's away, the mice will play means people sometimes misbehave when there is no supervision.

When the cat's away， the mice will play 意思是，当没有监管的时候，人们会做不恰当的事情，也就是通常所说的猫儿不在，老鼠成精。

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

阎王不在，小鬼跳梁。猫不在，老鼠成精。