Mixon you will need a tag and a group, and configure them so that only this group can write in this tag. The group can be the included Admin or Mods, but you can also create a new group.
In the permission tab of the admin panel, select "restrict by tag" and select the tag that will be exclusive for those "secret" discussions.
Then update the view/start/reply permissions under that tag to only allow your special group to view or post in that tag.