Which of the following is not typically used to detect intruders in a sensor system?

Microwave sensors are indeed used in some security systems, but their principal function is usually for detecting motion or presence based on the reflection of microwave signals. In contrast, the other options listed—infrared sensors, motion detectors, and video surveillance—are more explicitly designed to identify and track intruders.

Infrared sensors can detect heat emitted by bodies, making them effective for recognizing the presence of individuals in a protected area. Motion detectors are specifically designed to identify movement within a certain range, often used in both indoor and outdoor security applications. Video surveillance systems provide visual monitoring and can capture real-time activity, enabling security personnel to detect and respond to intrusions as they occur.

Given the context of intrusion detection, microwave sensors serve a more indirect role, focusing on motion detection rather than specifically identifying intruders, thus making them less typical for direct intruder detection in a sensor system.