How safe are adopted animals with their new owners?

Although we do our utmost to ensure that the people who adopt our rescued animals are appropriate new owners, we are always left with a niggling doubt as to how safe our animals actually are. A very worrying incident occurred last year which we would like to share with you. About five years ago we homed a beautiful dog to a couple, who came to adopt a second dog a year later. During follow-up conversations, they were full of praise for both dogs and we assumed that all was well and that no news was good news. Last year, I happened to glance at a notice board in a small supermarket and to my shock, I saw a notice offering two dogs free to good homes with photos of these two dogs. There was no date on the notice, but it contained the person's name and phone number. Of course I immediately recognised the owner's name and phoned her to say that the dogs were to be returned to us at once according to the adoption agreement which she and her partner had signed five years before and which specified that if they could no longer keep the dogs, they had to be returned to us and may not be passed on to other people or other animal organisations. The woman said that the notice was an old one which she had forgotten to take down, that her circumstances had changed and that she had already found a home for the one dog and that the second dog was no longer alive. After insisting on hearing the full story, she reluctantly admitted that she had taken the one dog to be put down at a private vet because he had become "nervous" once her child was born. She gave me the name of the vet, whom I immediately phoned and his receptionist confirmed that in May last year, the dog was brought in and he was asked to put the dog down. The dog was not sick. I asked her why the vet agreed to put down a healthy dog and questioned why he did not ask them from whom they adopted the dog? If he had scanned the dog for a microchip he would have noticed that the dog was adopted from Adopt-a-Pet. Why did the vet not think of phoning us to query the issue? So, to reconstruct what happened on that fateful day, one presumes that the owner/s got up in the morning, presumably had breakfast, opened the garage door, and called the dog to jump into the car (was his tail wagging because he thought he was going for a walk?) and drove him to a vet and asked him to basically kill our healthy dog. I say "our" dog because in our adoption agreement, which was signed by the owner and her partner, it stated that once the owner/s no longer wanted or were unable to keep our animal/s, ownership reverted to us, namely, Adopt-a-Pet. Surely the vets must think about these facts before drawing up a lethal substance into a syringe to kill a healthy dog who was only half way through his life? We would appreciate reader's comments to our question: "How safe are the animals which we adopt out to carefully screened new owners?"