I'm afraid that they do go missing quite often ... I now buy cheapest ones I can find and release them with a travelling companion, but I have learned the hard way, DO NOT RELEASE ANYTHING PRETTY OR CUTE OR USEFUL ... it will disappear.
Bear in mind, that in some (probably only a very few) that it could have been for genuine reasons that it was never dropped off again, people do have accidents, fall ill and even die sometimes, if no-one else in their family/circle of friends knows about Geocaching and these seemingly random trinkets, then they wouldn't know what to do with them.
Some are probably genuinely lost by accident ... not too long after my daughter and I joined, she found a gorgeous geocoin lying in the grass near a cache. If I'd parked my van 8 or 9 inches further onto the grass it would never have been spotted. The coin wasn't logged into or recently out of the cache it was close to, but once she grabbed it and explained where she found it, both the owner and the person who'd misplaced it were very grateful to her. The person who'd dropped it is a well established, regular and experienced cacher and he was mortified that he'd been careless enough for this coin to have fallen out of his pocket at the side of the road.