If it's going off randomly then my first thought is that you have a leaking sunroof or aerial which is letting water get to the central locking circuit board in the roof, above the mirror.
WilliamsClio has a guide for fixing it but it's a bit of a job! An alternative is to silicon seal the sunroof and the seals around it, but it's not very attractive... lol
If you want to turn the siren off then you should have a small key that fits in the lock that's underneath the plastic cover, next to the windscreen washer bottle. This turns the siren off but not the alarm itself.
To turn the alarm itself off you need to get the other small key that you should have and turn it off using the lock in the roof of the glovebox.
My solution would be to central lock the doors on the centre console when you open the door to get out, then lift the driver's door handle, push the pin down and shut it. It won't be alarmed but it will be locked, and the alarm won't go off randomly!
