Well, you've got a point there. In that case, the bag would be removed if you just disappeared. I've seen that done. Sitting at the gate, you just can't refuse to let people off. What happens after that may vary a bit.
Could someone just sneak off ? Probably. I'm not sure how thorough the screening of checked bags is these days. I've been retired for a good while and haven't paid much attention to such things.