Kyle, thanks for introducing a fascinating topic.
Nowadays, nearly all courses worth playing have been documented somewhere, and the possibility that someone would discover a noteworthy course is diminishing. In my life, I've "discovered" (in a sense) three courses I would rate as a 6. (Others, including as I found out later Ran and Tom, rate two of these courses lower.) By "discovered", I mean that they were not recommended to me by anyone in person, or via a magazine or book or this website. At least one of these three I sought out because I was interested in seeing an example of that particular architect's work. But moving forward, I doubt I will ever discover another 6 serendipitously, just because there is so much information available.
The question about being certain that one has played all the Doak 6s within a two-hour radius is quite relevant for me personally because I've just moved to a new area without a lot of golf tourism. The two-hour radius includes only a few entries in the CG or any other guide. So maybe there is a truly hidden gem lurking just around the corner. I recently found out that a nearby course will be closing soon. If I don't go and see it, it'll become an NLE Schrodinger's course. Most likely an NLE 3, but who knows, maybe an NLE 6.
Nearly six years after I took a trip to Scotland, I still wonder what might have been if I had stopped at an unheralded nine-hole seaside course by the side of the road, in hard-pouring rain. But it was a course I hadn't heard of, and we had non-golf sightseeing on the agenda that day, so we kept driving. In my dreams it lives on as a 7 when it's more likely a 3.