iPhone and Android app.
the app will need to live in the background.
the app will be able to auto detect if the user is coming near his car, or stepping away from the car.
once detected that the user is coming to the car, the app will ask some kind of a question, and based on the answer will perform a simple 'next step' action.
when the user step out of the car, the app will send some kind of a notification, either push notification/sound notification/or will launch with a certain screen.