It was the hood switch for me.

 

Mine would do that if I left a window down or even cracked by a hair. The motion alarm would detect the wind in the car or something moving and it would go off. That is never a good thing when you are in Trader Joes getting groceries and the dog is in the car

 

Pop your hood, and look at the latch by the radiator, you will see 2 wires, and there's your sensor.