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Also, make sure your battery is not below...what...10volts? It may just be an aftermarket feature, but I think my stock alarm used to go off when I had low battery power.
Definitely do what Casper suggested, unless you can find the shock sensor, then you can do it for free.
Also, be sure to change the battery in the remote. My girlfreind just had this problem where a dead battery in the REMOTE kept setting of the alarm.....