Security Issue

[Rickie3]

I'm hoping someone here can give me some advice,ok here goes
I created some private forums on my board that certain usergroups can view only
the usergorups that have access are
V.I.P colour Purple
Moderators colour Blue
Super Moderators Green
Admins Red.

Now my normal registered users are not allowed access to these forums at all unless i give them access

Normal registered usergroup colour is black.

Ok what happened some of the content from the private sections were taken screenshots of and given out to the public,which has made me very upset and very hard to pin point who and what member was responsiable.I have the hack installed that shows what member has viewed the thread.To my surprise a normal registered user was able to view that private thread,i have checked all my logs and that users permissions,and cant see how this member could view that thread,do i have a security issue on my board how could someone access pages without having the right permissions??? i'm at a loss
see my screenshot below the normal registered user who's name is in black and i have circled red should not have been able to access that forum at all,please can someone help.