Possibly another step towards solving this mystery (I'm sure you all care)...
One of my suspicions was that perhaps somebody clicked one of the Facebook "like" buttons in that forum, or that the Facebook cookie triggered by the existence of that button is reporting the URL to Facebook whether clicked or not (actually, come to think of it, it HAS to report the URL to Facebook to determine the number of 'likes' to display).
I found that the Facebook 'bot' had changed its nature since I originally defined it in the system, so I've updated it's definition in the system to allow for the change (and for some possible future changes). It is now being correctly identified again. Before I did this it would have been listed as a 'guest'.
I've still not seen any guests listed in that forum when I've looked, but I did just see this:
Screen shot 2011-05-11 at 2.17.24 PM.png
Screen shot 2011-05-11 at 3.45.13 PM.png
So I'll be interested to hear from others if they continue to get 'guest' appearances, and/or if the Facebook appearances now match up to the guest appearances they used to see (in terms of numbers and frequency).
So my current theory is that all the guests are merely instances of the Facebook 'bot' - which I have seen coming from several different IP addresses simultaneously on some occasions. If this is the case, the 'bot' would get nothing back other than a login page, as 'bot' access to these forums is not allowed, but according to phpBB, they would still be listed in 'Who is Online'.
PS.
According to Facebook, their bot will only visit a page if a user has provided them with a link. This implies that somebody has clicked on the 'like' button in a private forum, or otherwise that Facebook's idea of "provides us with a link" may mean having simply browsed a page with a 'like' button on it.
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