Glad your chickens are safe! My guess is that the owners didn't pay attention to the dead battery light on the collar. My dog won't approach our property line with or without her collar, but terriers will constantly test the system. Our neighbors were not good about replacing the battery for their dog until we reminded them that we now had a rooster and that we could not guarantee her safety when she came to visit.

Maybe my chickens are smarter than I give them credit. You won't find 1 chicken in my chicken run if a dog comes around in the yard.
I guess I prefer my chickens to be on guard all the time. Hiding out of sight is a good way to make other potential predators lose interest.