Catch somebody napping
Definition 1 of 1
Pro Tip 1/3Unexpected Action
Used when an action is done suddenly and the other person isn't prepared for it. 
The pop quiz caught the students napping, and none had studied.
Pro Tip 2/3Metaphorical Use
It's a figurative phrase and doesn't mean someone is actually sleeping. 
Our competitors caught us napping with their new marketing campaign.
Pro Tip 3/3Strategy Reveal
Can imply a smart or sneaky move by one party to gain an advantage. 
He caught us napping by selling the stock before the market crashed.