I have an old iPhone 3GS I don't use anymore, but I was thinking maybe it would still work great as a GPS tracker and bluetooth proximity sensor for my car. I use an iPhone 5s as my main phone. The idea is that whenever I walk to my car, the two phones will sense that they are approaching each other. When the two phones are barely within bluetooth range of each other, my iPhone 5s will start making a slow beeping noise. As the two phones get closer to each other, the beeping gets faster. Then once the phones are really close to each other, the beeping sound stops. When the phones sense they are moving away from each other, no beeping sounds are played. I tried googling for such a solution, but so far I haven't found anything reasonably close.