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