A security device adapted to mount within a ferromagnetic truss includes a sensing element, a first housing portion provided around the sensing element, a second housing portion positioned above the first housing portion and provided around a circuit board electrically coupled to the sensing element, a magnet provided on the first housing portion and operable to magnetically couple the security device to the ferromagnetic truss, and a retention clip provided on the second housing portion and operable to automatically secure the security device within the truss when inserted from below.