As near as I can tell, there is alost always a "guest" online.
I have wondered about this myself, and have concluded the following;
If your settings aren't set so you are automatically logged in with each visit, then you appear as a guest until you log in.
Also there is some kind of info gatherer called googlebot.com. I have seen this "guest" appear at the B77Experiment site, and I would imagine it is the same here. Some type of web site info gathering tool. If you have any number of tools installed on your computer, i.e. Gogle search bar, then the "bot" tries to gather info about the site you are going to. Of course the "bot" can't log on, so it appears as a guest.
At least that is my take on the whole thing.