Presence detection has always been a major pain in the arse.
The first usable system I saw was an echolocation mesh that was present in the building at the AT&T (nee Olivetti) Research Lab in the UK, where they had placed arrays of echolocation transducers in the raised ceilings of the floors of the building, which created specific harmonic signatures when they bounced off each badge. Its initial cost was in the 6 figures.
Pluto's approach made sense for its integrated proof of concept. Make the media directors clients, constantly pinging for the phones which were servers, advertising an RFCOMM service on a known channel (this was allowed on Symbian, however, as Hari discovered, the J2ME port required that the service advertise itself via SDP.). This created a semi-reliable way to make the system figure out whether you were in one room or another. This worked well in large houses because of the nature of bluetooth signals and their transmission classes versus signal attenuation, but in smaller living spaces, it was hell because the clouds would overlap, and it incurred so much logic in the system just to try and figure out where a device actually was, not to mention, the constant use of the bluetooth was hell on battery life, but it DID work, if you understood how bluetooth's signals propagate. However, as time has gone on, this method is much more difficult to do with modern phones, as they want developers to use wifi instead for "network like" things. We can still utilize this ,but it needs to be brought to the present reality of smartphones with multiple network interfaces. Not even mentioning the fact that the bluetooth interface was horrendously slow for how it was being used (30 seconds typically between binding phases, and typically 1 second between button presses to show the new screen), and the target was for phones with hard buttons only, no touch screen ability, and insanely low resolution (176x208 was the initial target.)
Fiire sidestepped this problem by making remotes with a definite mandatory access control address, and sending that address each time the remote connected with a target media director via its dongle. The other aspect was that the radio chips in these remotes had a rampable transmission power adjust, coupled with a directional antenna, which allowed it to quickly find a machine to bind to. It worked, and it worked well, but Fiire contracted with Gyration to build the custom device, of which only a limited run was made, and no more of these devices are available.
The approach of using a definite camera with a depth imaging sensor is the most reliable and cost effective option at present, as it costs $120 or so for a device, and the protocol for it has now been decoded and well understood. The data emitted by the camera is literally a 1 to 1 mapping of RGB pixel to depth, making this shit _really_ easy to package up and send to recognition engines. The DCE device just needs to be done!
Follow Me, is merely an event that is fired to the Orbiter plugin. Anything can fire it, and if you can definitely provide a PK_User, the system will do the right thing.
But please, forget the PIR nonsense. It will trigger far too many false positives at best.