Everyone's got their tell. The thing they do when they're doing something they're not supposed to be doing that gives it away that they're up to no good.
I've got this teenage boy, right? Since about age 2, he hums whenever he's doing something wrong. Doesn't matter how big or small the infraction; if it's wrong, he's humming. And it's the only time he hums; if it's just a happy-go-lucky thing he whistles, if it's wrong-doing, he hums.
It was so very funny that for years we'd immediately go to him upon hearing him hum; He was astonished that right away we knew he was up to no good. Finally he heard his Dad & I joking about it one day and realized that it was correct. Hasn't stopped him though. He still hums when he's up to no good.