Well found my old thread and more than three years later this problem has popped up again! This time it went off just when the door was opened (car had not been locked). The car is my moms now and has been for over a year. The other day my dad went out to put the baby car seat back in it and the alarm started blaring. It finally shut off, but today when I was going to take my mom on some errands in it, as soon as I opened the door (again, the door had NOT been locked), horn started blaring again, then it went off, but the lights kept flashing. Finally had to end up disconnecting the battery.

Guess will have to research again and see how I go about disconnecting it. Ugh.