It's probably spiders; spiders that do not have a User Agent entered in the settings in the CP show up as "guests" just like non-logged in users.
To check simply look at the "Who's Online" page and hover over the "info" box (the box with an "i") and it should say "User Agent" which will give some information about the "browser" the "user" us on (in the case of Bot's it should show what organization they belong to).
