American enough?

Before coming to Europe, I hoped I would be able to avoid any ugly American moments and that I could maybe help remove (or at least soften) some stereotypes of Americans in my classmates or other people around me. I really shouldn't have worried though-almost no one can tell my nationality. I've had one instance of "Uh, duh, of course you're American," which was apparently based on "the way [I] talk" and another when a guy on the train knew because I had my Nalgene bottle, and "all Americans have those." Other than that though, people can't seem to figure it out. I don't have any of the American accents they've heard on TV or in movies and my behavior doesn't give me away either.  If anyone does venture a guess, which is rare, they usually go with British. And almost everyone seems surprised when I'm American. It's interesting. I did give myself away to a guy on my floor by mentioning Halloween--that did it immediately.