The LS is smart, it knows you are the owner and that you were still inside the vehicle.
j/k
It's suppose to go of but ... I thought there was a time delay.
and by arming it, you should have double clicked the lock on the FOB and seen the red lamp on the dash flash. (PATS)
Try this ....
- lower the driverside window.
- close the door but don't lock it.
- from outside of car, double click the lock on the FOB.
- wait a moment. ensure red lamp on dash is flashing.
- reach inside through window, unlock and open door.
Alarm should go off, if not then there is a problem.
First GEN or second GEN ? (cuz we have the information to perform pin point tests)