A similiar thing happened to us. We had a Pure Bred Boxer named Duke, we used to put him on a very long type leash in our side yard (this was many years ago)We have about a three foot chain link fence around it, but it borders the street. One day we came home and the leash was lying next to the fence and no Duke ( he was our son's dog) We were devestated, we put ads in the paper, checked the animal shelters, etc. One day we got a phone call, my Dad answered the phone, and the man said he had our dog and for a price we could have him back. My Dad not knowing what to say, said could he call back when we were home, and the man never called again and we never saw Duke again.
That is why we never let our dogs down in the side yard alone, we always go down with them and sit in a chair while they play.