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 a warrant would be stronger, given that the government would have to actively affect your house. To play devil's advocate, the pot grower was emitting a signal in the FLIR case...

Anyone more educated than I able to cast light (haha) on this?

One nice thing about the Supreme Court is that every ruling comes with at least one (or two if the ruling is not unanimous) experts explaining the situation. And if any of the remaining Justices disagree, or feel that something was left out, of those two opinions, they can add their own.

In this case, the dissent wrote:

    Unlike an x-ray scan, or other possible "through-the-wall" techniques, the detection of infrared radiation emanating from the home did not accomplish "an unauthorized physical penetration into the premises,"

and:

    While the Court "take[s] the long view" and decides this case based largely on the potential of yet-to-be-developed technology that might allow "through-the-wall surveillance," ante, at 38-40; see ante, at 36, n. 3, this case involves nothing more than off-the-wall surveillance by law enforcement officers to gather information exposed to the general public from the outside of petitioner's home. All that the infrared camera did in this case was passively measure heat emitted.

https://supreme.justia.com/cases/federal/us/533/27/

Indeed. The ruling is quite clear that penetrating radio waves into the residence is presumed to be unacceptable. Even the dissent in Kyllo (FLIR) agrees on this point in regard to x-ray.

But consider:

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.

If you look at Gizmo's quotes, I think it's pretty clear they considered these kinds of technology tricks in deciding Kyllo, and 'taking the long view' that high tech collection of passively emitted information requires a warrant. I think the general approach is it's the result that matters, not the specific approach, e.g. 'expectation of privacy'.