I have a Mi fitness band and yesterday I lost it somewhere and it was already dark outside. But I know that I can connect to it via Bluetooth and it only has a limited range. So I walked where I was walking during the day and tried to connect. It wasn't in the house so I went outside. In the end I was able to triangulate it according to the speed of how fast I was able to download the data from it. It took some time in the dark but I found it and have it back ;)