Bluetooth beacons are what you call these. There's a few different app-based solutions, all of them proprietary as far as I'm aware, that connect to their brand of beacon regardless of its owner in order to update its position on the map. I feel like this should be possible to do in some way in an open manner, but I'd also be concerned about the security implications behind something like this. I wouldn't want everyone to see where all of my most irreplaceable items are, after all.