Sounds like your volumetric sensors (the things on the A-Pillar) are on the fritz!
Its a common problem due to them being sheeeeeiiiiiiiyyyyyt.
Apparently the way to fix them is by swapping the wires over, this allegidly reduces the sensitivity by ~25% and cures the problem.
I've got this problem with mine at the mo, just havent got round to fixing it yet - I've resorted to disarming the sensors at night.. otherwise as you've stated it goes off randomly!