In my '06 hatch the dealer installed a motion sensor as part of a recall. They installed it under the glove box and mine has a white knob that allows you to adjust the sensitivity. I had to turn mine down because the alarm would go off whenever the bus drove by my parked car.
The sensor (in my car anyway) is tucked up in the top of the footwell, I can see it if I get on my knees and look up there. It's got it's own wiring harness which disappears into the dash up behind the glove box. If you're still havin trouble finding it I can take a picture for you.
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