I'm wondering if maybe it's a spider that's not detected as a spider. A quick check of your Who's Online guests shows 10 of 16 randomly selected connections as MSNbot plus 1 Googlebot. Most all connections were looking at your chatbox archive.
You might try something like
this to cut down on the spiders being displayed on Who's Online.... or edit .htaccess to slow those bots down, or set your chatbox archive to only be viewable by members.