This sort of thing has been going on for over a hundred years on Dartmoor. They call it "Letter Boxing"

So in fact what we have here is an old english pastime caught up in geocaching! What a spiders web this thread throws up, and the wearing away of landscape by hundreds of people congregating round a particular spot, cows do the same around stones. The only difference are the cars and technology used nowadays, and what we consider to be "rubbish" left at sites..
The issue becomes less clear cut, except that treasure hunting in whatever form is seen as fair game and that rules have to be applied.