In my opinion, this would be considered neglectful ownership and abandonment. If it were a child left after hours in a daycare and no one could be reached staff would have to call the proper authorities. I would definitely call Rover to get some guidance around this but as it is negatively affecting your subsequent bookings I might suggest calling your local animal control and having them picked up. Let the owners pay for getting them out of the pound if they won't pay you for the extra time and inconvenience on your business. At least they will be safe and cared for and these owners will be required to answer for their behavior.