On the bottom of the of the main page of this threaded board, there is a section that says "Users Online".

Then this "Users Online" section will allow the people on the site to see how many Guests are online and how many Members are online.

Question is: Why do I see after Guests or Members sometimes a person who is logged in as a "Hidden" user? Who is this person?

Could be anyone. You can choose to not have yourself show up in the online list. Go to your profile button at the top, then click the account related settings link on the left. You will see an option on the page for "Show others your online status?" under the email section. You can check or uncheck that. :)

Oh, That was too simple. I was hoping for some mystery behind all of this but oh well. Kind of boring.