Or highspeed cameras being used to discern the minute colour and luminance changes on a window or a material in the target room which occur due to vibrations in the glass or many other materials due to sound in the target room. This allows reproducing sound that occurs in the room without having access to any microphones.
Or using appropiate lenses to "look through" blinds or other materials that have tiny lensing gaps which the typical person regards as not see-through.
