A portable authentication device for use with a security monitoring system for a building or a secured space within a building or other fixed property, the system including: a control unit for controlling, arming and disarming the security monitoring system; a detector (130, 290) to detect the breaching of an entrance giving access to the building or the secured space; a disarm node for disarming the system, the disarm node having a user interface to receive a user input to disarm the system, and a radio frequency transmitter to transmit, in consequence of the user input, a disarm signal to the control unit; the portable authentication device comprising: a radio frequency transceiver, a memory storing a portable authentication device ID, a motion sensor and a processor operatively connected to the transceiver, the motion sensor and the memory; wherein the motion sensor is configured to sense motion of the portable authentication device and to provide an output based on sensed motion of the portable authentication device, and wherein the processor is configured to determine whether the portable authentication device is stationary or in motion, based on the output of the motion sensor; wherein the portable authentication device is configured to receive a disarm instruction, via the transceiver, from the disarm node to transmit to the control unit a disarm request containing the portable authentication device ID; and wherein the processor is configured to transmit the instructed disarm request only if, when the instruction is received, the portable authentication device is not deemed to be stationary. a control unit for controlling, arming and disarming the security monitoring system; a detector (130, 290) to detect the breaching of an entrance giving access to the building or the secured space; a disarm node for disarming the system, the disarm node having a user interface to receive a user input to disarm the system, and a radio frequency transmitter to transmit, in consequence of the user input, a disarm signal to the control unit; the portable authentication device comprising: a radio frequency transceiver, a memory storing a portable authentication device ID, a motion sensor and a processor operatively connected to the transceiver, the motion sensor and the memory; wherein the motion sensor is configured to sense motion of the portable authentication device and to provide an output based on sensed motion of the portable authentication device, and wherein the processor is configured to determine whether the portable authentication device is stationary or in motion, based on the output of the motion sensor; wherein the portable authentication device is configured to receive a disarm instruction, via the transceiver, from the disarm node to transmit to the control unit a disarm request containing the portable authentication device ID; and wherein the processor is configured to transmit the instructed disarm request only if, when the instruction is received, the portable authentication device is not deemed to be stationary. a radio frequency transceiver, a memory storing a portable authentication device ID, a motion sensor and a processor operatively connected to the transceiver, the motion sensor and the memory; wherein the motion sensor is configured to sense motion of the portable authentication device and to provide an output based on sensed motion of the portable authentication device, and wherein the processor is configured to determine whether the portable authentication device is stationary or in motion, based on the output of the motion sensor; wherein the portable authentication device is configured to receive a disarm instruction, via the transceiver, from the disarm node to transmit to the control unit a disarm request containing the portable authentication device ID; and wherein the processor is configured to transmit the instructed disarm request only if, when the instruction is received, the portable authentication device is not deemed to be stationary.