Slug Taken, Cant Find Which Page

The slug is the poorly named post_name column in the $wpdb->posts table. Drop this into your theme templates somewhere– header.php, for example– and you should see any posts with a matching slug:

$q = new WP_Query(
  array(
    'post_type' => 'any',
    'name' => 'hello-world',
  )
);
var_dump($q->request);
var_dump($q->posts);

You can tweak the arguments for WP_Query if you feel you need to.