Heres a different assumptoon. a wire/rope clothesline to the opposing tree would also leave the same marks, especially with heavy items on it. could have even been ground dwellers that hung a line like that. I've sen all kinds of contraptions tied to trees.
seems it's more of an assumption that it may have been a hammock. But it does look suspicious.
So in the states and parks allowing hammocks, do the rangers now have to police your gear before you setup? A visit to your site after you setup is too late if you already rigged wrong.