I want a device that can either stick on dashboard or like a key fob in driver pocket that will help to detect whether person is in car or not. Requirements: - Device must have a detector and siren. When detector detects that person is not in car should buzz siren. - Can be a device in car or in person's pocket - Low cost and small form factor Solutions I can think of (please validate my understanding) 1. PIR Motion Sensor stuck on dashboard - Cons: The person need to keep moving 2. Accelometer and Gyroscope - Is it possible to detect whether person is walking using these sensors? 3. RFID? 4. Beacons? - Cons: Need a phone? What technology does smart keys for car use?