If non-registered users are clicking on private topics via widgets, you may be seeing this bug: https://bbpress.trac.wordpress.org/ticket/2279
I’m seeing the same behavour, private topics are visable to visitors not logged in.
I’ve uploaded Version 2.3-rc1 and the it did not fix the issue.
I’ve also removed the old widgets and readded them. No change in private posts visibility either.
The Topic View widget also shows private forums/posts to visitors.
