For as long as socks have been going missing, people have blamed it on the Sock Fairy (though it's probably a little more modern than that - stories about him/her probably started happening sometime after washing machines were invented). Usually depicted as an antagonistic male, no one knows for sure as he or she has never been seen. It is sometimes said the Sock Fairy will steal socks from the washing machine while leaving change in return but, really, who knows?