catch somebody in the act
catch (one) in the act
To see, and perhaps apprehend, someone as they are doing something (often something nefarious). The police caught the robbers in the act of ransacking another house. I suspected that my friend was stealing money from me, and then I caught her in the act.
catch somebody in the ˈact (of doing something)find somebody while they are doing something they should not be doing: She turned round to catch him in the act of trying to run upstairs.
This expression is often used in the passive: He was caught in the act of stealing a car.