Raspberry Pi 3, Raspberry Pi Zero W, and BT dongles plugged into older Raspberry Pis that don’t have BT on the board. These machines are already installed in these locations with other jobs so deploying the BT there is just because of opportunity. I’ve not deployed any specifically to get BT sensing capabilities.
I use sensorReporter or reelyActive to detect the presence of BT devices nearby. Unfortunately, reelyActive only sniffs for broadcast BTLE messages and most devices use a random MAC that cycles frequently so it isn’t much good for detecting specific devices (e.g. Rich’s iPhone is present). sensorReporter is fiddly and needs a lot of tuning to prevent false positives and false negatives.
For now, I’ve managed to get my network based phone detection reliable enough that I’ve actually turned off the BT sensors for now. I will probably go back to reelyActive at some point if I ever have the opportunity to deploy more Pis and what not around the house. I can then use them to keep track of what room, or even what floor our phones are on. I might be able to do something interesting with that information…