A structure intrusion may be determined. For example, a signal may be received corresponding to a wave propagating in the structure. Next, the received signal may be analyzed. Based on the analysis in a “passive mode”, a breach may be determined to have occurred in the structure when the received signal indicates that at least one aspect of the received signal crosses a predetermined threshold. Furthermore, based on the analysis in an “active mode”, a breach may be determined to have occurred in the structure when comparing the received signal to a baseline waveform indicates that at least one aspect of the received signal varies from the baseline waveform by a predetermined amount. The wave propagating in the structure may comprise an elastic wave and may be in an acoustic frequency range or in an ultrasonic frequency range.