That is how they did work at the time. The purpose is that it will be triggered if someone breaks the window and opens the lock from the inside...
Ok, that makes sense, I suppose. Still unsure about those ultrasonic sensors, I might have to just pull the pillar panels off and trace the wires.
EDIT: That also means I can fairly easily add (aftermarket?) remote unlocking, because it's likely operating separately from the alarm. Presuming there's a wiring plug hanging where the receiver module goes.