It seems like it should be possible to use wifi RSSI as a presence or motion detector. I have seen some very fancy devices that made low level use of the reflection information to target motion or people in a room, but the graph below is from an ESP8266 just reporting it’s RSSI every 5 seconds.
The device is sitting on my desk just a couple of feet away, so the range might not be very good, but still there might be instances where this is useful. You can quite plainly see in the above graph that I was in the office on and off until just after 7, and then it is fairly smooth while I was attending the local Philharmonic concert to hear my wife sing Mahler’s 2nd, and then picks up in variability as soon as I started going in and out of here this morning.
A lot of hysteresis would be necessary since there are many things that can affect the signal strength. Since it would take some time to know that you were there for sure it might not be useful to turn on lights or make adjustments when you arrive, but it could definitely be used to figure out when you’ve gone. Adding a Wifi calculated presence detection to such IOT devices would be very helpful. It would not work very well for battery operated devices however since you would need to wake up the wifi radios far more often than for the regular functions of the device.
Part two of this thought has code examples and is here.