Guests can be non logged in users accessing your site from a bookmark who aren't logged in, search engines trying to spider your forums; pretty much in your case users trying to access the forums who aren't logged in yet...

The best way to browse your guests:
http://www.yourforums.url/cgi-bin/ultimatebb.cgi?ubb=pntf_admin

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
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