I have a March 2006 SC430 (facelift version with LED rear lights and Bluetooth etc) and for the past few weeks the alarm has been going off randomly after being parked for 10 - 20 minutes. The strange thing is it doesn’t do it every day and even more strange is that it has done it 4 or 5 times whilst parked at work but only done it once whilst parked on the drive at home. It is still under warranty but I was just wondering if there is a simple solution to the problem (apart from not setting the alarm) before I make a fool of myself at the Lexus dealer. It may be going off when we park at shopping centres etc. where there is no one to let you know as we only know it is going off at work because people come and let you know as they know it is your car.

one thing that a few of us have found iworks is to close the air vents on the dash - sounds very wierd i know but it worked for me in the past on an IS and someone else with a GS

Check for newspapers, bits of paper, maps or umbrellas etc in the car - under the seats or in the storage nets.

Remove everything from the car might help...

These can move after the car has parked and trigger the alarm.

Another possibility is that a large vehicles going past may set off the alarm, I do believe that there is a screw on the alarm for sensitivity, although I have not looked on mine.
Hope one of the idea's work.
Ian

Had a similar problem a couple of years ago.

Turned out to be rain water getting through a cut in the rubber roof seal where it meets the top of the driver side 'A' pillar (due to poor windscreen replacement job). Depending on the angle it was parked at, water was sometimes being channelled down the pillar trim and dripping on to the alarm electrics somewhere on the side of the driver's footwell. Sealed off with rubber mastic, dried things off - sorted!

A bit more remote a possibility - but worth a quick check.

I took my SC on the ferry 4 times in the summer and I was surprised the alarm never went off. The car was basically bouncing up and down all the way and plenty of other car alarms where going off.

Mines a 2002 maybe it has a crappy alarm.