S.M.
I think that you have to assume you can't trust someone else's home. The problem is not what you CAN see (electrical outlets) but what you CAN'T see (medications, cleaning fluid, guns). Stay home for naps. Or if you must, bring a nap mat and put him down for a nap on the floor of the dining room and stay in the next room and talk quietly. If you visit somewhere regularly, like Grandma's, ask her to childproof a room for you. Mostly, enjoy the fact that he still takes a great nap and enjoy the time off at home.