In 1969 Army ROTC we were shown the AN/PPS-5 Ground Surveillance Radar. It was used to detect people moving around the perimeter at night. This was a live night time demonstration. It worked.
Young folks need to work with the older ones. You'd be surprised what we know that you don't.
Interesting article. It doesn’t say what frequency it works at. Here is an other interesting article about people radar. It would obviously have to operate a frequency such that the signal doesn’t get scattered or absorbed by the structure.