I currently have 5 private collections, 2 of which have the "hidden" attribute.
The other 3 collections are called: "remembered films", "submitted films", "ignored films"
These collections are supposed to be hidden, and indeed they do not show up on a film's collection tab when I am logged out.
So these collections should be marked as hidden in the user's private collection list:
http://www.criticker.com/collections/my-private