Which type of detectors can potentially "see through walls"?

Microwave detectors operate on the principle of emitting microwave signals and analyzing the reflections from objects to detect motion or changes in the environment. This capability allows them to "see through" certain materials like walls, as microwaves can penetrate many non-metallic surfaces. This ability makes microwave detectors effective for security systems, as they can cover a larger area and detect movement inside a room even when walls obstruct direct line-of-sight.

In contrast, photoelectric detectors rely on light to detect changes in their environment and would be unable to function effectively if obstructed by walls. Ionic detectors, which are used primarily for smoke detection, are not designed for this purpose either, as they respond to ionized particles in the air. Heat detectors detect temperature changes but require direct exposure to the heat source to respond, making them ineffective at "seeing through" barriers.