I doubt that there are too many things like pathways undiscovered in the gsmnp. What you saw was probably a manway and some dedicated, hardcore hikers use those to get away from the crowds and add a little challenge. I think there was an article a few months ago in backpacker magazine that detailed the 5 best manways in the gsmnp. These paths are usually very tight and cramped and, honestly, i don't see why anyone would really want to use them, especially use fisherman. I wouldn't recommend hiking these, though, as technically they are not marked trails i think, so i would imagine you could get into some trouble. Someone may know a little more about this topic because i'm not exactly 100% sure.