Catch is a very nice verb. You can catch the wind in a net, catch somebody red-handed and catch somebody's eye BUT nothing is so exciting as catching somebody with their pants down ;-)
If you are the master of catching people with their pants down it means you easily notice when the do something they don't want others to know about.
Definition: to discover somebody is doing something they shouldn't be doing.
Example: my grandma caught me with my pants down, as I was stealing cookies from the drawer...