Aliens. Must be. They've LEFT cutlery at my place ...
We had a BBQ with a couple of friends, they brought a salad with salad servers, no extra cutlery. When we were cleaning up afterwards, there were some extra forks and knives, not ours. We phoned our friends and they weren't missing any either, and when they were next at our place they said that wasn't their set! So I think aliens just take these socks and forks and things and zip around the globe, or maybe even to other planets, depositing them in people's houses and backyards, and having a grand laugh. My clip-ons are probably in Viet Nam.