Predefined electrical signals, produced by an electrical equipment in the building during a change of electrical operating state of said equipment, are detected on the building's power supply system. By analyzing each detected electrical signal, tracking data is generated including information relating to the date of detection, the electrical equipment originating the signal and the corresponding change in electrical state. At least one parameter representative of the activity carried out by said person is determined, in the form of a probability of an activity or a type of activity being carried out, based on tracking data generated during a predefined period of time.