For a deeper look please see: Mifare DESFire EV1
We have built an alarm system around it that uses different sensors to detect an possible theft.
This detection is configured very sensitive so that just a very short accleration other another abnormal activity activates the alarm system and you'll receive a SMS.
In addition to SMS ComCar Navigation supports Bluetooth Audio Speakers that can reach 120 dB or more.
Mifare DESFire EV1 4k costs about 3$
Bluetooth Alarm speaker that can be hidden somewhere in the vehicle. About 6$ Example Bluetooth Speaker
