I think that if you press the issue, you might find out something you don't want to know (I usually find that when someone say they found their dog a new home, the dog was actually shot). So whether you want to know the truth or not is up to you.
Yeah, it happens a lot around here, too. And they usually tell people they gave the dog away, especially if they think the person will object to their having killed the dog. So, can you make yourself believe he went to a nice new home, or will you worry less if you know he's dead? It's a hard thing.