If you have a contact for whom you don’t have permission to see, but that contact is attached to a case you can see, you can click on that contact but Highrise correctly fails to give you details (phone, address, etc) about them. However, if there is a case-note attached to that contact, clicking on the note will give you the full details of that contact. I think that’s a bug: you shouldn’t be able to use access to a note to gain access to a contact’s details in violation of the permissions scheme.