A displacement sensor for sensing displacement of a first surface relative to a second surface includes a coil mounted on the first surface, and encircling a region in plane; and a conductor extending lengthwise from the second surface through the region. The conductor has a volume that varies along its length. An AC source drives the coil to generate a magnetic field that pierces the region and induces eddy currents in the conductor. Eddy current losses resulting from the eddy currents vary in dependence on the volume of the conductor above and beneath the plane. A sensing circuit is coupled to the AC source to detect changes in eddy current losses, and thereby displacement. Detection of displacement may be used to signal tamper/intrusion. The sensor may further include a visual or audible output to signal such displacement.