Seems like we're each seeing different things. I don't see "Show unread posts since last visit" just "unread".
Ah, if you have shrunk your header, that would be invisible. You're probably one of those header shrinker people, aren't you?

Top right: there's a little up-arrow or down arrow. If you put your mouse on it, it says "Shrink or expand the header." Little arrows like this appear in a few places on the forum, so you can hide stuff you don't need, and especially so you can accidentally click on them and never figure out why things vanished.