We track both the last time you visited a forum and the most recent post that you've read in a forum.

IIRC, the lightbulb will go out once you exit the forum, but will go on again if people post to that forum after you're visited it the first time.

This is generally because people tend to read things inside a forum that they've just entered rather than backing out and doing something else.

Hrm... Mark all read didn't quite fix it for me. It worked for a day or so, but today, I had a lightbulb on with no unread posts inside. Installation forum I think.

When I enter one of my forums and get out again it sometimes marks some others as 'read' too.