It would seem that despite the 'All cookies cleared!' message when logging out, that is not the case.
After clearing cookies manually it is possible to access forums and view posts. Until you have been logged in.
Timmcat, I was thinking something not too dissimilar. It appears that even if you are not logged in, your presence is known by cookies left behind. I have learned enough lately to know that Saab Dastard, helpful as he is, might not be in a position to know everything that goes on.