I didnt know if theres a topic for this, so i made one! 

Im curious about our guests. Every time I check whos online, I see guests viewing their messages, sending messages and viewing whos online - among many things that members can do.  So how is it possible for guests  to do stuff members can do? 

They're just search engine spiders browsing easily generated links like:
?action=pm
?action=who
etc.

Even though they may not have access to anything AT that URL, they can still access the URL itself. But can't do anything from there.