IT's just a hunch. Do keep in mind that with tropo, sites don't need to be strong or on a 500' tower at the top of a mountain. As such, a site pulled via extreme tropo could be a not-high-profile site and might not even be pulled in one or two counties away under normal circumstances.
Often when I would see tropo from my old Toronto location, it would be tropo to Eastern / SEWV -- I'd pull in the sites like they were 5 miles away from me, but I wouldn't pull in all of the sites between me and them. I love tropo. Hopefully somebody will figure out where the sites are. I think it's a good bet that they aren't more than one or two states away from WPA/EOH, for whatever that is worth. So I guess that leaves places like IN, KY, MI, Canadia lol, NY, NJ, MD, VA, DC. I'm sure somebody could narrow that down by just saying "hmm, where would there be a 4-5+ site federal P25 site setup that has gone undocumented so far". And the answer to that would probably be in a major metro area that also has significant federal presence.
Mike