Which alarm have you got? Scorpion or Meta
The scorpion alarm should tell you why the alarm is going off with the led on the dash flashing. When you put the key in the ignition the led flashes - Straight from the manual
1 flash for interior sensors
2 alteration vehicle reference angle
4 Boot or bonnet opened
5 Doors opened
7 Attempt to start the car
It does these flashes 3 times.
I've also had a couple of problems with my alarm going off randomly.
- 1st one was a dodgey cable to the siren, think it was earthing so making the siren go off (this didn't show any leds flashing)
- 2nd was a mini connector/fuse for the interior ultrasonic sensors. The connections were broken, my own fault as I had pulled them appart by accident but not realised, but when clicked back together they just touched. Led would flash once indicating something to do with the interior sensors. Thought it might be a moth for a while

Good luck