! So, I just found this situation where a private topic can be visible to guests. Yay, I win. The board list will show a link to it (although not actually browsable) if it's the last updated topic in the board. I've decided to 'fix' this by forcing a query_see_topic, and if the topic is private, do an extra query (which shouldn't be TOO slow, but I'm no specialist) that will return the proper last viewable post for you. This will only happen if the last post is private, so it shouldn't be too much of an issue. (Subs-BoardIndex.php)

@ Feedback is warmly welcomed!!