Did this happen to anyone else's pet?

I'm sorry you lost your dog. That's the hardest thing to go through. The problem is, dogs can't talk to us to tell us what's wrong so vets really have to guess a lot of the time. If there's a mystery it's hard for a vet to figure out because they can only treat the symptoms they see. You should have done a necropsy if you wanted to know but now the only thing you can do is move on. Remember the good times you had with her. And definitely let the boarding place know she got sick there.