Electronic Car Key
Generally we park our car and lock it through our remote electronic locking device. When you leave the parking place on your work, you some times wonder whether you locked your car or not. There is no way to know unless you walk to the car and lock/unlock it once again.
My suggestion is the remote key should have two LED's of color red/green. When the car is locked, green LED should glow and vice versa. Thereby we can know whether car is securely locked or not just by looking at the key. This does not involve much technology, as LEDs can be lighted through potential free contacts by using power of key battery.