Curlytails, this is an excellent question. I understand that there definitely is a difference between a "responsible breeder" and the "backyard breeder", puppymill, and petstore. While a responsible breeder will require a contract with a return clause/mandate, sadly, the other three definitely outnumber the responsible breeders by far.
Like you, I've never seen a contract from a breeder, but to your question:
My question is… does a breeder have any way of enforcing or following up on the contract if they ever found that they were NOT notified before a dog was given up? Or, say, if a pet was sold on a spay/neuter contract, and that didn't happen, and the dog was instead being used to purposely breed puppies for profit?
I think it was a rescue group that took a dog from Ellen deGeneres when she lovingly gave it to a friend… even with all of her resources, Ellen could do nothing to get the dog back. So, I think that if a breeder would diligently follow every dog that was created within their control, and if they found that this dog was no longer with the original owner... I think the owner could pursue it, if desired. And if the breeder could afford a lawyer to pursue it, if desired.
What I really admire about Basenji Rescue and Transport (aka BRAT), is that they not only require a background check for their dogs, they do a background check for the potential owners. They make sure that the new owners are financially and conscientiously responsible citizens, and that the veterinarians that know these potential owners consider the potential parents as "good parents". BRAT puts a contract out there that mandates you MUST return the dogs to them, no questions asked! And finally, BRAT will test these dogs for Fanconi so that the new owner has an idea of the future health costs, as well as microchipping them so that IF(not when, but IF, lol)... IF these basenjis would flee, there would be a lifeline for these basenjis to go home.
(I think of Bunny/Sophia, and while she didn't know it, she was definitely a benefit of BRAT. Kudos to BRAT to getting the word out, and to getting volunteers to help in her search… but I digress, lol. )_