Yeah I've got my old subs everywhere. I mounted one on the front of my buddies lawnmower at his workshop to pick up any stray screws, scrap, or random crap. He says every now and then he'll check it and it will be full of screws and scrap steel.
Good idea for key retrieval, just make sure the magnet is nowhere near your wallets or phones. Bye-bye credit cards and phone.