An optical fiber is monitored for changes in a monitor signal caused when the fiber is subjected to vibration, motion, or handling typical of an intrusion attempt and for triggering an alarm condition in response by one or more sensors. The sensor emits data on each disturbance event including the existence of the disturbance event and a value associated with the disturbance event dependent on an intensity of the disturbance. The arrangement herein provides an algorithm to analyze the disturbance events which calculates for each event a score dependent on the presence of the event and the value associated with the event, generates a summation of the scores in a FIFO and when the summation of the scores exceeds a predetermined threshold within a predetermined time period, the processor emits a signal triggering the alarm condition.