I'm not sure but I think this might be where a post is deleted for any reason, eg spam, but the cookie in your browser files is not updated.
I reckon Lee has reported this to the engineers, but I will alert him anyway.
Mark
reading posts and refreshing page UPDATED SINCE LAST VISIT is still displayed.
"mark all as read" and refresh page sometimes UPDATED SINCE LAST VISIT disappears for a while...NO new posts...and then UPDATED SINCE LAST VISIT appears again with NO new posts .

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic