[stomachmonkey]

People that dump pets usually take the collars off.

Do a 411.com reverse look up on the number.

If it has changed recently it may still contain previous persons name/address.

Post flyers.

As Steve said take the dog to a vet and have it scanned for a chip.

Use a vet local to where the dog was found. You might luck out and get the vet that the owner uses.

Possible that the owner is on vaca and had someone sitting the dog and it got loose. (happened to me once with one of my cats, my sister let it out by accident)

Take care of it for a couple of days.

The owner will turn up and if not a purebred Yorkie is not difficult to adopt out.