Earlier quoted context omitted.
There's already been a ruling saying this isn't ok: http://en.wikipedia.org/wiki/Kyllo_v._United_States Kyllo was about thermal imaging but the opinion is quite clear. There's a much, much stronger case for thermal imaging equipment than there is for a device which probes via radio waves.
Just a few thoughts. Very simplistically... FLIR is a special camera that detects infra-red light. IR is electromagnetic radiation that is emitted from objects with heat. RADAR emits a more "traditional" radio wave, and then detects objects when the radio wave reflects back. Unlike FLIR (which is passive), I would consider RADAR to be active. I would think (rather naively so) that the case against using RADAR without…
Your house is constantly saturated with radio waves from TV stations, radio stations, wifi etc.
What if I were to design a device that passively received reflections of these signals rather then generating it's own to achieve the same function as the device in this post.
So I now have the same result, but I am "merely" receiving reflections, rather then making my own.
There is no difference in the function, this is rules lawyering at its best.
I am willing to let police observe emissions on visible wavelengths ONLY because that is a slippery slope to not being allowed to look into windows.
Anything you need a device to view is not ok to view without a warrant, no matter the technique.