"Unknown Action" is more of a technical term to describe what Infini said in that the forum software does not have a means of knowing what is going on because it is not defined standard; most release candidates for SMF have all actions covered, but when modifications are added on, possible new actions are not understood because they are new and not recognized by the forum software as a whole. The "Nothing, or nothing you can see..." is a defined term because the forum software has it set for actions based on usergroups; i.e. viewing a board or actions in a board that are restrictive to usergroup.
Then again, there are occasions where guests will show up on the Who's Online list as "Nothing, or nothing you can see...", so I am not sure what that is about. The boards that are off-limits to guests may give them that term for some reason when they try to view them; search bots are inquisitive.