I had a violin disappear after a sim put it down, a garbage pile turn invisible after I dragged it outside, and a couple of other items just vanish. They were still there, but couldn't be seen or used. I knew they were there because I couldn't put anything else on the spot they were supposed to be.
I got them back by going into build mode and moving items and rooms around, or by creating small new rooms over where the items should be. They either reappeared in the room, or were put into the family inventory.