"A new technology can inconspicuously scan the same surface [a blank wall] for shadows and reflections imperceptible to the human eye, then analyze them to determine details, including how many people are in the room—and what they are doing." #privacy https://www.scientificamerican.com/article/a-blank-wall-can-show-how-many-people-are-in-a-room-and-what-theyre-doing/