I think Disney generally does a very good job of attempting to locate, and return, lost items. Of course human nature is what it is and there's always the possibility that someone steals something. But most of the time, things just get lost and Disney does their best to find them.
Some years back, my son lost his beloved stuffed puppy, which he takes with him everywhere to this day, apparently on a ride at the Magic Kingdom. We assumed it was gone for good. He was devastated. We reported it, but didn't think much would happen. We later were notified that the Disney staff had found it. It had apparently dropped down in an area of the ride track or whatnot and got covered in grease. Not only did Disney find it, but they professionally cleaned it and shipped it back to us.
Disney is not always the bad guy.