[catch] {sb} in the act

to see or arrest a person while they are doing something illegal or wrong

The idiom "catch someone in the act" has its origins in the concept of witnessing or discovering someone in the midst of committing a wrongdoing or engaging in prohibited behavior. The phrase emphasizes the act of apprehending or observing someone red-handed, providing undeniable evidence of their actions. It conveys the idea of catching someone in the very moment of their transgression, leaving no room for denial or escape.

The homeowner caught the thief in the act as he was trying to break into the house through a window.
The teacher caught the student cheating during the exam, confiscating the cheat sheet and reprimanding them.
The security camera footage caught the shoplifter in the act of stealing merchandise from the store.
The vigilant neighbor caught the vandals in the act of defacing public property and reported them to the authorities.
The boss walked in unexpectedly and caught the employee browsing social media instead of working, leading to a reprimand.