Is there any chance that some new radio transmitter has started operating in the vicinity where you keep the car? Radio station, TV station, amateur radio operator, etc? There is a place in San Diego, Soledad Mountain, where many people go to view the city from high up and also to see the huge cross on the site. When they are leaving the parking lot, the signal for their fob to unlock the doors and disarm the alarm system gets swamped by the TV transmitting antennas very close by. When the owners use the key to open the doors the alarms go off and they can't shut off the alarm until they start the car with key. It's a real riot hearing so many alarms going off all day long. Here is a link to a story in a San Diego freebie newspaper that touches on this subject:
http://r.search.yahoo.com/_ylt=AwrBT...AmDadpc2QClAY-