Place it right after the surprising action, then follow with Ceeday Huh, Oh My God Wow, and No No Wait Wait when the person tries to explain. Keep it as the opening question in a short reaction chain, with the follow-up sounds answering it in turn.
Über diesen Sound
This is a short spoken question, 'what are you doing?', lasting about two seconds and delivered as a direct challenge to someone caught in the act. It lands with a mix of confusion, suspicion and playful accusation, as if the speaker has just walked in on something they were not expecting. That plain, questioning tone makes it feel instantly familiar and easy to picture in a real conversation.