Sorry to be so harsh, but its a sore point for me, as Ive had my skyline since i was 17, and I got zero respect from anyone due to other people on the road being tools.
Did you get the alarm installed or whas it already in the car? has it ever done it since.?
My alarm occasionally goes off for no damn reason, when its locked in the garage. Could have just been one of those things to file under the 'WTF?' pile.