The iPhone is small and easily misplaced. I've left mine in restaurants more than once, but was lucky to notice when I got in my car that Bluetooth never activated so I could make a quick recovery.
Last Alert Pro sends an audible alert when your battery drops below any of three pre-set levels. If you're not near your phone, you'll get an email as the phone drops in battery strength. The emails will give you a street address of where your phone is, and a link to a Google Map.
The app also provides what it calls radar tracking. The app can be configured to send a second email to another device running the app (iOS or Android) showing your phone's location and provides very specific walking instructions as you approach it.