To me the moral of the story is to include a notice that the user has only logged out of this (the auth consumer's) site, but may still have an active session w

To me the moral of the story is to include a notice that the user has only logged out of this (the auth consumer's) site, but may still have an active session with auth provider X. That exact language probably isn't appropriate, but it's at least the information a responsible auth consumer should try to convey.