prendre quelqu'un la main dans le sac

to catch someone red-handed

This expression means to catch someone in the act of doing something wrong, typically stealing or committing another offense. It's equivalent to the English 'caught red-handed'.

Le voleur a été pris la main dans le sac en train de fouiller dans le tiroir-caisse.

The thief was caught red-handed going through the cash drawer.

Think of a pickpocket at a medieval fair, caught with their hand literally inside someone's bag. Documented in French literature by the early 19th century.