In what some call the "First Holocaust" the Nazis had a program to kill off the mentally ill and other "useless mouths". Among the many problems they encountered was the people involved blabbing about what they were doing. The public at large became quite vocal (for Nazi Germany) in their opposition to what was happening.
When it came time for the Jews and all the others rounded up in the selections to be murdered it was Himmler who insisted that only his SS could be trusted with the job, as only they had the discipline to keep their mouths shut. IIRC there was even an occcasion where he spoke to the SS at Auswitz, telling them among other things to be proud of what they were doing and that it was `a page of glory, never to be written in the book of history`. I may have that last quote a bit wrong but that is the gist of it.
While I don`t think you could call it a secret there was most definately an attempt to conceal what was really happening.