A monitoring device (MON) 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 (CNT); a wireless transmitter (TX) configured to be automatically switched between a default sleep mode and an operating mode; and a processor (PROC) for controlling the monitoring device (MON); the monitoring device (MON) having: an armed mode, during which the time-out counter (CNT), 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 (CNT) is disabled; the processor (PROC) configured to: switch the monitoring device (MON) 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 (CNT) using the wait time value when the presence detector (MVMT) detects a presence; and return the monitoring device (MON) to the unarmed mode when the time-out counter (CNT) times out; wherein the processor (PROC) is further configured to: switch the transmitter (TX) from the sleep mode to the operating mode once the time-out counter (CNT) has timed out, said time-out indicating that the space has been un-occupied for the past wait time; and return the transmitter (TX) 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 (CNT); a wireless transmitter (TX) configured to be automatically switched between a default sleep mode and an operating mode; and a processor (PROC) for controlling the monitoring device (MON); the monitoring device (MON) having: an armed mode, during which the time-out counter (CNT), 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 (CNT) is disabled; an armed mode, during which the time-out counter (CNT), 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 (CNT) is disabled; the processor (PROC) configured to: switch the monitoring device (MON) 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 (CNT) using the wait time value when the presence detector (MVMT) detects a presence; and return the monitoring device (MON) to the unarmed mode when the time-out counter (CNT) times out; switch the monitoring device (MON) 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 (CNT) using the wait time value when the presence detector (MVMT) detects a presence; and return the monitoring device (MON) to the unarmed mode when the time-out counter (CNT) times out; wherein the processor (PROC) is further configured to: switch the transmitter (TX) from the sleep mode to the operating mode once the time-out counter (CNT) has timed out, said time-out indicating that the space has been un-occupied for the past wait time; and return the transmitter (TX) to the sleep mode at least after having transmitted a message indicating at least that the space is un-occupied. switch the transmitter (TX) from the sleep mode to the operating mode once the time-out counter (CNT) has timed out, said time-out indicating that the space has been un-occupied for the past wait time; and return the transmitter (TX) to the sleep mode at least after having transmitted a message indicating at least that the space is un-occupied. <EOS>