A motion detector for detecting a tampering within a detection zone. A vibration-sensing unit sensitive to vibrations applied to the detector is provided to produce a vibration output signal representative of a detected parameter of the vibrations. A light-sensing unit sensitive to light in a predetermined spectrum is provided to produce a light output signal representative of a detected parameter of the light in the predetermined spectrum. A processing unit is further provided to determine if the vibration output signal is consistent with a predetermined model indicative of a tampering and producing an activating signal accordingly.