It gets more and more strange...
I find the available forums literally with
Code:
$dbres = $db->fetchAll('SELECT node_id FROM xf_forum');
ie. pure SQL, not even some special XF api. And I just checked that all sorts of permission-limiting doesn't change the fact that each forum has an entry in xf_forum.
And I even use it myself with forum areas that guests can't see...
Do you have entries in that table for 42/53?