A security assembly includes a first building structure at least partially defining a building opening. The first building structure has a first surface. A movable building structure is movable between a closed position in which the movable building structure covers the opening and an open position in which the movable building structure uncovers the opening. The movable building structure has a perimeter with a second surface disposed in opposition to the first surface when the movable building structure is in the closed position. An optical sensor apparatus includes an electronics module mounted in association with the first surface or the second surface. The electronics module has an optical emitter and an optical receiver. The optical emitter emits a first beam. A reflector arrangement is mounted in association with the other of the first surface and the second surface. The reflector arrangement provides a plurality of sequential reflections of the first beam to thereby produce a second beam directed at the optical receiver.