Thank god you saw it, and she didn't come to any harm when she was loose either. I'm paranoid enough about my dogs, I can't even imagine if something like that happened. Our vet told us about a client he had with a well-bred boxer, and he had a chainlink fence with a combination lock on the gate, had him chipped, did everything he could think of. That didn't stop the thieves, they cut through the chainlink and stole his puppy, and it wasn't seen again.
Build that fence as big as you can, and either run a hotwire a foot out from the fence, or an invisible fence, so the dogs won't cross the line even if someone breaks through the fence. It makes me so mad that people will do this. It takes a dogfighter or someone equally as heartless to steal someone's pet; nobody who loved an animal would wish the loss on anyone else.