catch (one) napping
To exploit or capitalize on one's inattention (or, sometimes, literal sleep). We were able to sneak into the building because we caught the security guard napping. I scored a goal after I caught their defenseman napping.
catch somebody ˈnapping
(informal) find somebody not prepared or not paying attention, and perhaps gain an advantage over them as a result: Chelsea’s defence was caught napping in the final moments of the game when Jones scored his second goal for Liverpool. Nap means ‘sleep’, usually for a short time and especially during the day.