How to sniff out a rat
 

I have a private moderator's forum. It is protected by both permissions and a password. I have reason to believe that some who shouldn't have access to it, do have access somehow. I've always been careful to mask moderators who have been let go so they can't even see the forum any more. The only way someone could be in would be if one of my current mods has given out his username and password and the moderator's forum password. Any way to solve such a mystery?