Once when my husband and I had returned downstairs after out usual Sunday afternoon shenanigans, our then four or five year old daughter informed me that she'd been outside our bedroom door. "I thought you were having heart attack," she said. Great. You leave the children, what you imagine is well entertained downstairs, whilst you go up for a "sleep" because you're "so tired", and it turns out they're outside your door listening to every sound.
And then years later you discover that your younger daughter and that mischief, her brother, regularly entertained themselves in this fashion.