Android: Fitness trackers are awesome until you lose them. Bluetooth Finder helps you find lost Bluetooth devices if they are nearby. The app works with peripherals that don't keep a constant connection to your phone or tablet -- which means it isn't great at finding headphones and other devices that usually stay paired.
I tried it with my Fitbit and it worked great, telling me how close I was to the device within a few feet. The reviews warn that it doesn't work with all devices, so your mileage may vary. The app is free so you've got nothing to lose except maybe the device you're trying to find.
Bluetooth Finder (Free) [Google Play Store]