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Are they visible for anonymous users? They should be visible for logged in users with enough privileges, but not for anonymous users...
It could have been that you have overriden AccessDenied() in the template base class
I am finding that expired pages, by setting the stop publish date via the CMS editor, are still visible to the user. Although they disappear from page menus on the view mode, they can be accessed via the page url. I would not expect this to be possible.
Is there something else I ought to be doing?