Meet the Davek Alert Umbrella: a smart umbrella that alerts you when you are about to leave it behind. There is a Bluetooth-based proximity chip in its handle that broadcasts a silent signal to your phone. If the distance between the chip and your phone exceeds a certain distance, the umbrella sends you a reminder to your phone.
The Bluetooth LE chip has a coin cell battery which should last up to 2 years. The battery is replaceable. You can get yours here.