I've been informed by those who know much more about Harry Potter than I that Hogwarts is protected with a Disillusionment Charm so all muggles will see is ruins with a "Danger: Keep out!" sign around them. (Apparently this was in one of the first three books, but I'm not sure where it was.)
My first thought was, "Well, a danger sign would encourage some muggles to investigate!" Which leads to what I think is an obvious question: Will a disillusionment charm on a place like that still work when a person is real close to the location? Will it work from inside the location?
For example, if a curious muggle made it to the gates of Hogwarts and started walking around inside, would they still see it as abandon and in ruins, or once they entered, would they see it as it is?