If I could correct you: Microwaves do not need a metal surface to be reflected. Any EM wave is partially reflected when travelling into a medium that has a different refractive index - eg a human body. Although I agree it is FULLY reflected from a metal surface.
Secondly, if I could comment on my use of these RCWL devices, I found them very susceptible to 2.5GHz emissions - ie those emitted by the CPU in the wifi band. I got very fewer spurious detections if I separately mounted the RCWL about a meter away from the ESP cpu. This could be a good reason why the newer mm microwave sensors which operate at ~60GHz, will be less susceptible to wifi signals.