Note 3/5 placed along the mystery line:

I do see something there on Maxar, Esri and Bing imagery. I drew the lines when I was mapping the nature reserve. 

I see a couple of options: either some underground installation, an overland route swept away by erosion, or an incomplete construction project. Considering how it is interrupted by water flow patterns, it makes me think it's something pretty old - perhaps a historical road construction that was abandoned, but I can't be sure.

In any case, my hope is that one day someone will go there and be able to figure out on the ground what this is and then add the correct tag. Leaving the lines now makes it a) easier to do, because it is already mapped - just needs the tag, and b) more likely to happen, because it draws attention to it. 