If you have a WiFi system in your house then it can be used to tell when you or more people are present and even locate where you are down to a particular room!
The general idea is called Device-free Passive Localization - DfP and it is a set of techniques used to locate a person without that person having to carry a tracking device of any kind.
It sounds like magic but it relies on the simple fact that the human body is basically a large sack of water and this changes the electrical properties of the space it occupies. This causes changes in the way radio signal propagate and if you can detect the distortion in the field then you can use this to work out if a human is present or not and perhaps even pin down the location.
If you have ever tried to listen to a VHF radio that was tuned to a weak station then you may have noticed that the reception varies as people move around relative to the aerial. This is exactly the effect that makes DfP possible but its one thing to curse the loss of the radio program as people move about and another to turn it into a reliable technique of working out where the people are.
A group of Egyptian researchers have managed to make a system - RASID (Robust WLAN device-free pasSIve motion Detection) that uses off the shelf WiFI access points to tell when a house is occupied and even which rooms are occupied - and all it needs is the right software.
First the system learns what the fluctuations in the radio signal are like when there is no one in the house. Then it goes into its detection phase when any abnormal changes in the signal strength is used to detect a human entering the house.
If the house has more than one WiFi access point, and to achieve an overall coverage this isn't uncommon, the system can use the relative changes in the signal to determine where in the house the human is.
(b) Two Persons Moving
If you would like to see a more general presentation of an earlier DfP system by the same researchers then have a look at this, slightly overlong video:
What might you use this sort of system for?
The most obvious use intruder detection. Any house or office that has a WiFi system has a ready built burglar alarm that just needs some software to make it work. It could also be used in a smart house mode to switch power on and off when the owner returns or leaves. There is also the slightly more worrying idea that the software could be installed covertly to monitor what the occupants of a house were in.