This happened to me - the problem is the META alarm system gets re-set when the battery is disconnected, and becomes a bit sensitive to inputs... Do you have the key fob??
From the manual:
"The volumetric protection (ultrasound) can be disactivated(sic) by keeping the button on the remote control unit pressed for approximately 3 seconds. Confirmation of this feature is indicated by 2 flashes of the hazard light circuit followed by 1 fast, 1 normal, 1 fast."
Alternatively, you can remove the side panel from the centre console on the passenger side (the one with carpet on it - 2 screws hold it on), and disconnect the alarm unit by reaching in and pulling out one of the 2 big plugs (a bit fiddly to do this).
If you have the key fob and it works, the alarm should be off when you unlock the central locking?