this sounds over-broad. if i make a tape measure yagi with some PVC pipe that is tuned for ~100mhz, and i tune to an FM station that is "over the radio horizon"^ and aim it at a patch of sky where planes travel, if i receive the remote FM radio station at my location that means it's reflecting off something.

This is the most banal passive radar you can make. There's also one that doesn't require "over the horizon", but does require two receivers, you need two directional antennas at the same wavelength (two identical yagis will do, or if you're clever, two 9wl:0.25wl off-center fed dipoles), one aimed toward a radio source, and the other aimed at your desired "radar area", you can correlate signals on the radio-side to the radar-side.

So because i typed this, does that mean black helicopters later for me?

^"over the radio horizon" for VHF/UHF is a function of transmitting antenna height, relative to your location, and is usually "line of sight, plus 10%", assuming no tropospheric ducting. VHF/UHF are not like lower frequencies that are reflected by the ionosphere (sometimes) and the "ground" (sometimes), their range is drastically limited.

so in essence, if you know of a station in a nearby county or whatever, but you have never received it at your location, even with sensitive radios and good isolation (>=15dBd), and there's no physical barriers between those two points, and you aim a sensitive antenna and receiver at that transmitter, if you do receive "snippets" of signal - something is reflecting it.

this stuff is on various websites, archive.org, probably wikipedia.

If you have a VHF receiver of any sort, that allows external antennas, you can measure out nine wavelengths of wire, as straight as possible, aimed slightly (a degree or two, depending) off center from your target area; and 1/4th wavelength of wire in line with the other, and attach the short one to "ground" and the long one to "antenna", you now have a ridiculously cheap antenna. It's easier to make and set up than a beverage antenna, as well.

note: mods, delete this if i violated any rules, i don't see how, but i'm no law-thing