Ah, yes the jokes on them & that is pretty funny. Another way...
I've done something of similar sentiment but rather than waiting for them to leave, I waited for them to return, re-enter the vehicle & buckle up. Not being shy, I walked up to the car, opened the door while they sat there wide eyed and confused & said 'You dropped this Sir'. Then, not so politely I returned their discarded food containers to express my displeasure at their lack of consideration.