Alarm going off for no reason - Avensis 2006 d4d

[avensis2006d4d]

Happened twice today where my alarm will go off for no reason. It's so loud that i'm tempted to bring in all my stuff from the car and leave it unlocked for the neighbours sake. 

The Battery is fine at 12V.

There's a button up by the in-car light switch that's supposed to turn the alarm off but I don't know how to use it properly, the manual is quite hard to understand on this. I press the button and then the red light on the lock display above the cd player starts blinking red very quickly for a while, does that mean it's off? 

Any help will be much appreciated

[flash22]

Does the alarm use the horn or a sounder ? If it's a sounder it may have a Battery backup that has gone bad, if the alarm uses a bonnet switch check that its not faulty/corroded

[Mike Cross Reading]

My understanding is that there is a fault whereby when the roof of the car becomes too hot from standing in the sun, the alarm goes off.

You can prevent this happening simply by pressing the button to the right of the interior lights in the front top of the cabin before you close the door and lock the car. The button merely switches off the part of the alarm that senses the cabin, so the alarm is not set off.

You need to do this only on a hot sunny day when the car is not in the shade.

The cure is supposedly to insulate the inside of the sensor unit from the heat of the roof but I have tried to do this twice and failed.

[Canard]

I had exactly this problem on my last (MY2000) Avensis.  I understand that a tired old Battery can cause this problem but I had just replaced mine, eliminating that possibility. The alarm would go off randomly day or night and got to the point that I had to disconnect the Battery every time I parked the car. In the end I accessed the plug and disconnected the siren.  This was quite a palaver as the unit was hidden somewhere on the offside of the engine compartment underneath rhe washer reservoir and the headlamp unit and various other bits and pieces had to be removed first.The siren unit can suffer from corrosion which is apparently the cause of the problem.  You probably have two choices, either replace the part or just leave it disconnected.  I went for Plan B as I didn't think a rusty 20 year old Avensis would figure too high on your average car thief's hit list. In the case of my car, there was a handy You Tube vid explaining how to do it and normal relations were soon restored with my neighbours.