Startup Idea : Warning Smartphone not in car

My wife often drives away without her smartphone. I could use one of those small devices for finding things, but then you need to remember to charge them. I think it would be useful to have a usb car charger that included something that would detect the absence of a smartphone and make a noise or blink a light. By building it into a charger there is not need to remember to charge it or have something extra to carry around.