Husband clumsily dropped his radiator cap into the engine compartment, today, and insists it will not cause any problems (plans on replacing the cap but not finding the dropped one).
I sure wouldn't worry about it, especially if it dropped behind the engine. That area on my car has swallowed tool or two, most recently my 12 mm 1/2" drive socket. If it's not in the very visible belt area on the right (passenger) side of the engine, there's nothing for it to interfere with.