I believe if you do research you will find this event was disclosed in 1946.
http://www.history.navy.mil/faqs/faq20-2.htm 'Slapton Sands: The Cover-up That Never Was'
"Nobody ever lifted that order of secrecy, for by the time D-Day had passed, the units subject to the order had scattered. Quite obviously, in any case, the order no longer had any legitimacy particularly after Gen. Eisenhower's Supreme Headquarters, Allied Expeditionary Force, in July 1944 issued a press release telling of the tragedy. Notice of it was printed, among other places, in the soldier newspaper, Stars & Stripes.
With the end of the war, the tragedy off Slapton Sands -- like many another wartime events involving high loss of life, such as the sinking of a Belgian ship off Cherbourg on Christmas Eve, 1944, in which more than 800 American soldiers died--received little attention. There were nevertheless references to the tragedy in at least three books published soon after the war, including a fairly detailed account by Capt. Harry C. Butcher (Gen. Eisenhower's former naval aide) in My Three Years With Eisenhower (1946).
The story was also covered in two of the U.S. Army's unclassified official histories: Cross-Channel Attack (1951) by Gordon A. Harrison and Logistical Support of the Armies Volume I (1953) by Roland G. Ruppenthal. It was also related in one of the official U.S. Navy histories, The Invasion of France and Germany (1957) by Samuel Eliot Morrison.
In 1954, 10 years after D-Day, U.S. Army authorities unveiled a monument at Slapton Sands honoring the people of the farms, villages and towns of the region "who generously left their homes and their lands to provide a battle practice area for the successful assault in Normandy in June 1944." During the course of the ceremony, the U.S. commander of the North Atlantic Treaty Organization, Gen. Alfred M. Guenther, told of the tragedy that befell Exercise Tiger."
Just took one google search