I don't think it's a dump, not with a tag but a disconnected number -- why dump a dog with any collar, never mind tag? Especially around Christmas, who's that evil? Bet the dog slipped out of the house and people are just too busy/crazy to have noticed (here's a scenario, it's grandma's dog and grandma got a stroke for Christmas while EMS is rolling her out the door little dog darts out, all sorts of shit happens to retarded little rat dogs).
Humane society, SPCA or yorkie rescue (most purebred rescues will also take purebred crosses). Rescues are good when it comes out to checking out people who apply to adopt.