That's not a fair analogy though. I agree that I wouldn't expect every page on twitter to be protected. But if you find a direct link to a random document indexed by Google, then check and the page that links to that document is protected, I personally anyway would assume the document itself was exposed accidentally. Obviously not everyone agrees though, which makes the reasonable person thing difficult to decide.

As for the lock on the door comment, I'd say it's more like if you noticed a store is left unlocked in the middle of the night, and therefore assume you're welcome to go in and walk around. In fact, they probably didn't leave it unlocked as an intentional invitation.