Super Secret Area - Dead Ahead!
I happened to go to the index. I then noticed the icon, and went into the topic to check the latest replies. They were dated while I was away on holiday.
I usually just hang around the index and check boards I'm interested in when there's a new posts icon but I don't read every thread in those boards so I'm in favor of the first option. There does seem to be a bug I've been getting for a while though, where there are new posts in some boards but the new posts icon doesn't appear on the index. Dunno what's up with that.