"Room occupancy sensors" are a common product. Butlr apparently uses a low-rez IR camera, although they avoid calling it that. Passive infrared is common. Milli

"Room occupancy sensors" are a common product. Butlr apparently uses a low-rez IR camera, although they avoid calling it that. Passive infrared is common. Millimeter wave is available.[1] [1] https://www.alibaba.com/product-detail/IoT-Commercial-People-Counting-Device-Occupancy_1600908375222.html?spm=a2700.7724857.0.0.78890EOm0EOm5J https://www.alibaba.com/product-detail/IoT-Commercial-People...