We present a novel technique for detecting objects that are concealed by textiles. Items of virtually all materials
can be sensed, which includes metals as well as dielectrics. Our technique detects the acoustic phase lag of
objects when they follow a driven oscillation. The acoustic phase is measured interferometrically with continuous
wave radiation at about 77 GHz. Typical oscillation amplitudes are about 1 μm, which is close to the human
perception level. The technique provides no insight into the concealed item's material properties. It exclusively
shows whether or not an object is hidden, which is the most relevant information for many sensing applications.