Lock all the doors (except the drivers door) whilst inside the vehicle, then get out and lock the drivers door with the key. That arms the alarm for the doors, as said above, but doesn't arm the motion sensors. I had a similar problem with my disco 300tdi 1998 reg, the sensors on the passenger side pillar had got damp when the sunroofs were leaking in and it kept setting the alarm off at random.
Check the headlining for damp patches or moisture and dry them out, this may work (it did for me).
p.s. even if you dont have sunroofs, apparently you can still get leaking through the alpine windows and roof bars.