Few of us like to stalk people to see what they're doing. The best way to watch over the members is to check that... or at least that's what they like to say. XD
But really everyone who stalks that page opens a tab on the online list because it auto-refreshes. After 15 minutes of doing nothing (I believe it's 15 mins, it might be 10) PC will "kick" you off and you will no longer show up on the online list. Now when it does that all threads are marked as read. Since I don't want this to happen I open a tab on the online list and it auto-refreshes and so I can be on PC all day and the threads don't get marked.