Car Alarms: keyless entry panic problem, 2003 toyota sequoia, home security system

Question
My 2003 Toyota Sequoia after engaging the lock, will randomly set off the panic mode on its own and the horn starts honking.  It may not happen for a day or it happens all the time.  It could be 5 min. after or 12 hours after.  It could be raining or it could be beautiful out, so weather hasn't really come into play.  I brought it in to be looked at but they said they can't check anything unless it is happening while it's there. I can't imagine they can't look at something. HELP! I do have Sirius in the car, could this be causing some interference. The only time it doesn't happen is if I don't lock the car.

Answer
Don't think Sirrus will interfere here.

It can be something as simple as a wireless
home security system that will interfere, a
radio tower near by with a stronger signal etc...

Is it always the same location?
Do we know if it's after market or factory?

Need more to go by to try to help a bit more.

ALARMTEK