4Runner False Alarms

My son's '00 4Runner with the factory installed remote keyless entry
and alarm (per TSB EL008-02 description of key FOB) is giving false
alarms. The alarm goes off and no one is around the truck. This
usually happens late evening. When the remote is pressed to unlock,
the alarm horn sound four times and then unlocks the door.

I haven't found any troubleshooting information in the body electrical
or diagnosis sections of the factory service manual. Google searches
haven't found anything useful. Over the weekend I'll try to figure out
what the 4 chirps means when you turn off the alarm. On other alarms
it is a signal pointing to the trigger that set off the alarm. If I
can figure out which trigger is giving false alarms, the search will be
much easier.

Anyone know where to start looking for this type of false alarm? Is
there a motion sensor or glass
breakage sensor somewhere that can be adjusted?

I'm baffled, please help??!!

Paul.
Phoenix, AZ