couch cushions, between furniture and the wall, on really low shelves in bookcases where I never look, in the toddler pottys (lift the bowl out and in the bottom), cabinets, in toys, toy boxes, in drawers, the list goes on and on.
Also on a side note I found one thing that the kids had taken off with hidden in the fire place. Found it just a month ago when we were cleaning out the fire place to burn one of the chimney sweeping logs to get ready for a real fire.
Basically any nook and cranny in your house is a potential hiding spot for a toddler. They will find a way.