ProxAlert is a Pebble app developed for Hack the Planet 2015 in Mountain View, CA by Ty Morris.
The purpose of the application is to provide immediate alert to a Pebble user who leaves the vicinity of their phone without explicitly denying its aid.
The impressive component of this project (in my opinion) is the technology required to make it work, rather than the product itself. It was very much a hack job. None of the traditional distance-measuring tools were at my disposal, so in order to determine when a user leaves their phone I had to finely tune the communication methods provided by the (small) Pebble API to use packet latency as a measure for distance. The result is essentially a reconstruction of the ping system using a pebble watch and android phone as two nodes connected only through Bluetooth (which has no distance or wave strength accessors!) I found this hack to be very rewarding when it finally worked.