byebyegti: If the doors/bonnet/boot/fuel cap aren't shut properly, your car won't arm - someone's already said that. That button by the drivers door will turn off the interior monitoring, pressing it again will turn it back on - you can check by hanging something from your interior mirror, swing it, lock the car, wait 30sec and then the alarm should go off.
You mentioned something about your alarm going off, and I think the 2 are related. My rear washer hose cracked a few months ago, letting washer fluid leak into the boot lock, this led to false sensor readings in the car - it thought the boot wasn't shut properly because of the water in it so, after leaving the car a while, the alarm would go off.
I hope that makes some sort of sense. Oh, and if I'm right about the washer, and you want to get it fixed, you can do it yourself - http://www.golfmk4.co.uk/
A member of this site runs that website I think.
Hope I helped and not just rambled!