Catch someone doing something you think they shouldn't can mean:
- You're feeling watchful or suspicious right now somehow in your life
- You feel you have power over the person because you have a special knowledge or understanding of them (or wish you did)
- You want to see that person punished or get into trouble, perhaps because you feel they've been dishonest or unfair
- You're feeling the need to monitor others or take responsibility for their actions, or it can represent nosiness, vigilance, over-vigilance, or inserting yourself into others' business