It's in this code area in index.php
// If guest access is off, a guest can only do one of the very few following actions.
elseif (empty($modSettings['allow_guestAccess']) && $user_info['is_guest'] && (!isset($_REQUEST['action']) || !in_array($_REQUEST['action'], array('coppa', 'login', 'login2', 'register', 'register2', 'reminder', 'activate', 'help', 'smstats', 'mailq', 'verificationcode', 'openidreturn'))))
{
require_once($sourcedir . '/Subs-Auth.php');
return 'KickGuest';
}
elseif (empty($_REQUEST['action']))
{
// Action and board are both empty... BoardIndex!
if (empty($board) && empty($topic))
{
require_once($sourcedir . '/BoardIndex.php');
return 'BoardIndex';
}
// Topic is empty, and action is empty.... MessageIndex!
elseif (empty($topic))
{
require_once($sourcedir . '/MessageIndex.php');
return 'MessageIndex';
}
// Board is not empty... topic is not empty... action is empty.. Display!
else
{
require_once($sourcedir . '/Display.php');
return 'Display';
}
}