The alarm goes off, but think about this:

When i bought my alarm a friend of mine (kind of a shady one) bet me that he could get in my car without the alarm going off and i said sure. He coat hangered the chrome lock switch on my car and then readied himself, he quickly opened the door, popped the hood and yanked the positive lead off of my battery, all before the alarm got through it's warning beep sequence, i went back to my shop that day and bought a back-up battery.

With two people in under 20 seconds thieves can:
Remove all cd's/radar detectors/LCD Screens/laptops/any other loose valuables in your car
Remove any CD player not bolted onto the car
Remove any non mounted amps/subs in your trunk
And in most cases (as most security systems do not have battery backups) just pull your positive lead from your battery and disable your alarm.


You will realize what is going on in around 15 seconds
spend another 15 thinking about a course of action (do the thieves have guns etc...), and spend prob 15sec to 30sec gettin you butt out the door, by the time you get near your car, they will be gone, with all of your goodies.

-GSRacer