What about multipath? Is that not an issue at wifi wavelengths? Or is that a sub or superset of reflections? It was quite a long time ago now that the first "proof" that one could use leaky WiFi to "see through walls" and observe people moving around inside a building from without.

Participate in a transmitter hunt (also called a foxhunt or a t-hunt) where the organizers or the people hiding the transmitter know their stuff. Reflections and multipath can lead you miles away from a transmitter location.

Anyhow, someone asked me if I knew how to do this without consent once; that is, if I knew how to track people in a building without them knowing. This was 8 years ago or so. I had hoped saying "that's not possible" would dissuade them, but instead they just never spoke to me again.

Oh yeah, multipath is the term I was thinking of. As I recall, the issues were that a "longer" path may have a stronger signal than a more "direct" path, and that a straight line of sight path is not a given.