I have had similar problems with Ford trucks in the fleet I manage. It is possible that a door switch is failing; these things are tied into the GEM, which controls the alarm system. No good way to disable the factory system without the owner's manual, and maybe a scanner for the car. The door switch is a micro-switch, and can send false signals to the GEM if dirty, or the latch is badly worn, or if they're in a bad mood. The switch should be inside the door, on the door latch.