You may help the next person who has a yard
I am ALWAYS watching my girls in my fenced in yard - My neighbors would never steal my girls - but I take NO chances with them.
It only takes a second for a dog to go under a fence.... and they could get hit by a car, stolen or Mauled by another dog....
you're VERY smart to be there with them. That just SUCKS those people would do that - they HAD to know she was yours - and it sure sounds like they were going to just keep her - how LOW IS THAT !
I had to redo my whole fence at the base because my neighbors dogs were digging - and it really worked - I just put up baseboards so NOTHING can get under it on that side.