In a homebrewer's rite of passage, last night I poured the oxyclean/etc out of my flask and, whoops! dropped a stir bar down the drain. (I usually remember a keeper magnet but forgot this time. Yes, beer was involved). Anyway, this morning I realized I could stick a neodymium magnet on the outside of the P-trap and slide the stir bar all the way back up to the top where I could grab it with a different neodymium magnet. Problem solved!