Been killed twice by 2 separate people hiding in the wall of the prison building.
Same place 2 times. If you go in the prison and upstairs and in the room with the 3 bunk beds it is the wall behind the two beds next to each other.
When you walk in the room you cannot see the player until they shoot you then their player is revealed. They appear to be coming out of the locker to the right of the bed nearest the window.
Have not managed to get back to a prison to try and recreate this yet.