A monitoring device for detecting whether a space is occupied, comprising: a presence detector (MVMT) configured to monitor an occupation status of said space; a time-out counter; a wireless transmitter configured to be automatically switched between a default sleep mode and an operating mode; and a processor (PROC) for controlling the monitoring device; the monitoring device having: an armed mode, during which the time-out counter, pre-initialised using a locally stored wait time value, is configured to count towards time-out; and an unarmed mode, during which the time-out counter is disabled; the processor (PROC) configured to: switch the monitoring device from unarmed mode to armed mode either: at a locally stored start time; or at a time when the presence detector detects a presence; during armed mode, re-initialise the time-out counter using the wait time value when the presence detector (MVMT) detects a presence; and return the monitoring device to the unarmed mode when the time-out counter times out; wherein the processor (PROC) is further configured to: switch the transmitter from the sleep mode to the operating mode once the time-out counter has timed out, said time-out indicating that the space has been un-occupied for the past wait time; and return the transmitter to the sleep mode at least after having transmitted a message indicating at least that the space is un-occupied. a presence detector (MVMT) configured to monitor an occupation status of said space; a time-out counter; a wireless transmitter configured to be automatically switched between a default sleep mode and an operating mode; and a processor (PROC) for controlling the monitoring device; the monitoring device having: an armed mode, during which the time-out counter, pre-initialised using a locally stored wait time value, is configured to count towards time-out; and an unarmed mode, during which the time-out counter is disabled; an armed mode, during which the time-out counter, pre-initialised using a locally stored wait time value, is configured to count towards time-out; and an unarmed mode, during which the time-out counter is disabled; the processor (PROC) configured to: switch the monitoring device from unarmed mode to armed mode either: at a locally stored start time; or at a time when the presence detector detects a presence; during armed mode, re-initialise the time-out counter using the wait time value when the presence detector (MVMT) detects a presence; and return the monitoring device to the unarmed mode when the time-out counter times out; switch the monitoring device from unarmed mode to armed mode either: at a locally stored start time; or at a time when the presence detector detects a presence; at a locally stored start time; or at a time when the presence detector detects a presence; during armed mode, re-initialise the time-out counter using the wait time value when the presence detector (MVMT) detects a presence; and return the monitoring device to the unarmed mode when the time-out counter times out; wherein the processor (PROC) is further configured to: switch the transmitter from the sleep mode to the operating mode once the time-out counter has timed out, said time-out indicating that the space has been un-occupied for the past wait time; and return the transmitter to the sleep mode at least after having transmitted a message indicating at least that the space is un-occupied. switch the transmitter from the sleep mode to the operating mode once the time-out counter has timed out, said time-out indicating that the space has been un-occupied for the past wait time; and return the transmitter to the sleep mode at least after having transmitted a message indicating at least that the space is un-occupied.