A known method for sensing presence of a human being at an occupancy location is based on passive infra red (PIR) motion detection. A PIR detector can determine start of presence at a location, but it can be difficult to establish exactly when the end of presence has happened. An approach that has been taken is to measure for presence periodically, i.e. a PIR motion sensor is pulsed every so often to check for presence. If presence is detected within a fixed duration, a presence signal is sent to a monitoring server. If no presence is detected in a fixed duration, a no-presence signal is sent to the monitoring server.

In a first aspect, this specification describes a method of operating an apparatus to determine presence and absence of one or more individuals in an occupancy space.

The apparatus may comprise a processing apparatus having a number of modes. In each mode the processing apparatus is responsive to at least one of a signal from a sensor and expiry of a timer. In one mode, the processing apparatus may be responsive to both a signal from a sensor and expiry of a timer. In another mode, the processing apparatus may be responsive to expiry of a timer only.

The processing apparatus may be configured to store data in first and second variables, the first variable being adapted to store an indication of whether the processing apparatus responded to a signal from a sensor or to expiry of a timer, and the second variable being adapted to store an indication of whether, at an earlier time, the processing apparatus responded to a signal from a sensor or to expiry of a timer.