Check out the screenie, how is this guest (necro)posting? 

This is someone or a bot that typed the URL to access the post form by hand. If someone types http://www.omnimaga.org/index.php?board=2.0 in their address bar it will show them trying to view the trash bin forum and people who have no access to it will see "Nothing, or nothing you can see". Even if they get an error while trying to access content they're not permitted to, the online list still shows that they tried to do that last action.

On Cemetech front page you even see new posts in private boards, but cannot access them.

Most likely, or a guest browsing boards without going through the board index.

When I compiled yealy board stats for most TI forums a few months ago, I guessed post IDs to try finding the last post of every year, so if a staff browsed there they most likely saw me viewing the staff forum a few times, even though I couldn't access it.

Note: Omnimaga got 4 staff boards (one which contains the 2009 contest submissions and admin instructions on how to add sub-forums, but is never used otherwise, another just for coders of tomorrow and admins, another which irc chan ops have access too as well and the trash bin, which only admins can see )