Went and checked with the other woman and it's not her yorkie, as her pup died in August.
The animal shelters in my area haven't had any reports of missing yorkies. My neighbor will put up some flyers tomorrow, but they said they're keeping the pup if they can't find the owner (my neighbor said the pup is exactly what they're looking for, as his daughter is nuts over my yorkie Cookie). I brought over some Royal Canin for the pup to eat while he's at my neighbor's and their young daughter just wants to feed it to the pup by hand (but her father is smart enough to keep her away from this new dog for now, as of course we all know nothing about him).