A guard sensor injects a multi-GHz (multi giga-bit) guard signal along a guard signal transmission path above an observable knee in the amplitude response of the path to define a guarded region and to detect physical or electromagnetic intrusions of that guarded region. At frequencies above the knee, the signal transmission path exhibits increasingly non-linear and even chaotic behavior that improves the overall sensitivity of the sensor and its ability to detect slight changes in the distributed transmission parameters that characterize circuit devices, signal paths and signals. The guarded region may be used to protect a combination of circuit devices, physical connections, interfaces, high and low frequency signal transmission paths and signals.