WELCOME!! What a cutie!!
Of course, you have run an ad and/or contacted the human society to report you have found a lost dog, right? You don't have to turn dog in to human society, but most humane society will write down description of dog, etc, in case owner calls them looking for this dog. Sooner or later, most humans looking for their lost dog will call the humane society.
Many papers will run "free" ads for found dogs.
If it were me, i'd also put up posters, in area where i found the dog, "found a dog".
as well as read newspapers and posters for anyone looking for this dog.
The pup should also be checked for any microchips, too. (the humane society or your vet can do this, takes all of two minutes, is probably free to do this)
If it turns out that the owner of this pup IS looking for the dog,
and wants it back,
do not despair,
there are millions of homeless puppies and dogs looking for a human who wants a puppy or dog:
http://www.petfinder.com/index.html
GOOD LUCK!! KEEP US POSTED!!!