A station is provided with an external industrial container-shaped shell having a window therein for admitting the passage of a monitoring beam therethrough. The preferably infrared beam is initiated by a beam transmitter having a lens aimed at a corresponding beam receiver in a second station, the stations being adapted for employment in pairs. Interruption of the beam by either an intruder passing between the pair of stations or tampering with a barrel causes a signal to be transmitted to a remote alarm. The housing is designed to be inconspicuous so that an intruder will not locate the station and thereby avoid detection.