Around 2003/2004 a local magazine called NAG (http://www.nag.co.za/) had an opinion piece at the end in which the author theorized using beacons for an inverse purpose: attach Bluetooth fobs/beacons to your keys and then triangulate given how your phone scales the signal strength of the BT signal. IIRC it would have only worked with Nokias because no other brand implemented this Bluetooth power consumption optimization spec. I do remember the article indicating that such beacons could be found with a little bit of effort.

The only piece missing from the puzzle was that phones only have one Bluetooth aerial, so triangulation would have been impossible. It would still be possible to determine unitless distance.