When I check active viewers, it often says there are guests reading and replying to topics. This can't be so, because my forum is a members only forum. It can only be read by members.
Because its not a members only forum in the strictest sense. You haven't enabled "Require Registration" you have used private forum to secure each forum on your board. This means that anyone trying to view a forum will get the "You must be logged in" error. These users are not logged in (they are guests) but technically they are "viewing" the forum (in reality just the error message).
Enabling "Require Registration" should prevent stop the problem.
But is says they are viewing a specific thread. It can't be the top thread that shows up on the default page, because I noticed they are farther down in the forum.
Same principle. When users try to view the following thread they are counted as viewing the thread (technically they are viewing the thread) but do not actually see the text of it. They get an error.