irishluck:
I think I got it. It senses IR changes in its path.
I just did a search as far as how far it can detect. Read somewhere that it can sense up to 10m?
Does that sound correct?
Yes, most sensors have a detection-range of 5-10 metres, thats the reason why i said that you should not place it at a bad position, because then it could detect people just walking around the car & falsely triggering the alert everytime.
Also you could think about setting up a key-switch for disabling the siren after an alert, otherwise you would have to press the reset button on the arduino everytime (which could be difficult if you place the arduino in some type of box).