I just finished an out-sourcing Android app job which targets at Bluetooth Low Energy (BLE, BT4.0) tracker. You can see my demo video in below link.
This work shocked my client because I made it in a very very short time. On the otherhand, they out-source the iOS version to another developer who uses Swift to do his work. And I'm almost twice faster than him.
In this video you can see there are 3 BLE trackers (a device which simply has one button on itself) provided by my client. After app scan and connect to those 3 devices it setups 3 peer to peer connection. App can send beep signal to devices and vice versa. When any one of the devices is out of range, your app alarms.
Beyond that there is special function: When selfie mode is enabled, you can take selfie photo by clicking the button on tracker. I like this function pretty much.