Walls and dark illumination conditions are becoming less of a problem to identify and infer the presence of people and even actions taking place "out of our sight".
Nice work by Prof. Dina Katabi (MIT Computer Science & Artificial Intelligence Lab, MIT-CSAIL) on "Making the invisible visible":
1) Tianhong Li, Lijie Fan, Mingmin Zhao, Yingcheng Liu, Dina Katabi, "Making the Invisible Visible: Action Recognition Through Walls and Occlusions"
https://arxiv.org/abs/1909.09300
PDF file of the paper available HERE.
