I'm using evernote on Windows 7 and on an Android tablet (Nexus 7).
Oh, I see - my notes that I didn't think were in any notebook are in the catchall one called "kenatsun's notebook". So I was able to remove my note from the wrong notebook by moving it (by drag-and-drop, on Windows) to "kenatsun's notebook". Thanks, s2sailor.
BTW, is there any way to put the same note in more than one notebook? I don't mean by making a copy; I mean a single note that is in multiple notebooks, so any edits to it show up no matter which notebook you access it from.