Well the hidden user on at the moment is me! I had to ask John about this the other day because I didn't know either, but in your profile, you can choose to hide your online status. In my case its so that people dont realise how much im on here - i have it on all the time when im working - which is all day!!
yeah u make me look bad cos ur online all day but im the only one showing up cos i do the same when im in my room