I agree with the fact that if they no longer want the dog they must agree to give it back, but really thats all you can do. You can't make them tell you the whereabouts of the dog at all times. I understand you have the dogs best intrest at heart but its unrealistic. I don't think the police would do anything if you called them and told them the people you sold a puppy to moved and didn't tell you about it. Why don't you do something like the breeder we got Mercedes from does every once in a while she has a picnic and all the puppies from her litters are invited to meet and she gets to see the dogs again

edited to add: Other breeders copy people's birth certificates!! No wonder identity theft is so easy!