Since upgrading to IPB 4.0 I've noticed a lot of my users seem to be looking at a forum and not actually doing anything, so I logged in with a second account on a different browser, went into a topic and checked - it still said that I was looking at the forum index. I left it a good 5 minutes and it never updated. However, when I went back to the forum index it updated immediately and then seemed to track the topics I was viewing.

Anyone else noticed this?

We noticed, that some forums are mentioned in the Online User list and some not. I couldn´t find a reason for this yet. Is there a setting, which blocks forums/toipcs to be shown?

Maybe there is a correlation with banned users? Some forums can be viewed by banned users.

Now, the group for banned users is not allowed to see any forum. It looks like, there are all forums shown now in the online list. Maybe it's the same problem in your forum? @markymids

It only updates your location once every 2 minutes or so on IPS 4. So if you went to the front page and then 5 second later go to topic, you'll still be viewed as being on the frontpage. 

However if you then refresh the same topic 115 seconds later (if it's 2 minutes), then other people should also see you as active there.