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X-Ray backscatter is definitely used for this, most famously by secret NYPD vans. https://www.theatlantic.com/politics/archive/2015/10/the-nypd-is-using-mobile

• transpute 5y ago
  > The radars work like finely tuned motion detectors, using radio waves to zero in on movements as slight as human breathing from a distance of more than 50 feet. They can detect whether anyone is inside of a house, where they are and whether they are moving. The cost of those devices should fall with 802.11bf Wi-Fi. > the vans deliver a radiation dose 40 percent larger than delivered by a backscatter airport scanner; bystanders present when the van is in use are exposed to the radiation that the van emits… there may be significant health risks associated with the use of backscatter x-ray devices as these machines use ionizing radiation, a type of radiation long known to mutate DNA and cause cancer. Could this radiation meter detect the presence of such a van? https://www.gqelectronicsllc.com/comersus/store/comersus_viewItem.asp?idProduct=5637 https://www.gqelectronicsllc.com/comersus/store/comersus_vie...