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.
Apple offers the free Find my iPhone app, which is handy when you know your phone is missing. Last Alert Pro (US$1.99) is a bit of a twist on the Apple solution.
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.
Of course Find my iPhone is a great utility, but you have to know that your phone is lost. Last Alert Pro is proactive, and tells you where the phone is as its battery drops.
I tried the app and it worked well. There are test modes for each feature. You can set the battery to any levels you want for warnings, and you can specify up to 3 email addresses for alerts. When the app emailed me, it had the exact address where my iPhone was. The app has to be running in the background -- which takes a measurable toll on your battery life -- but for forgetful people that issue will be worth the peace of mind of knowing if you've left or dropped your iPhone somewhere.
Last Alert Pro requires iOS 6 or above. The radar feature doesn't work on an iPod touch because that device does not have a magnetometer.