Retro-reflection is a light pattern in which an object sends incoming light back toward its source. In practice, camera lenses, corner reflectors, and some engineered surfaces can create this return signal, especially when viewed from the right angle and under controlled illumination.
In cyber security, retro-reflection matters for physical counter-surveillance. Hidden-camera scanners use it to spot lenses inside ordinary objects, because a lens may flash differently from plastic, glass, or metal trim when light is swept across a room. That makes retro-reflection a useful clue in room inspections, but not proof on its own: mirrors, glossy packaging, and other shiny items can cause false positives. Defenders use it in handheld inspection tools, phone-based scanning apps, and optical filters that help separate lens-like reflections from background clutter. Attackers can try to reduce the signal by hiding the lens behind dark covers or choosing angles that avoid a strong return.