I wish to categorically state that I did not work for German military intelligence during World War II.
So I’m not a primary source. I got this trivia tidbit from Commander in Chief: FDR’s Battle with Churchill, 1943 by Nigel Hamilton, which I am currently reading.
Morrison’s report had only whetted the President’s curiosity aboard the C-54 Skymaster as they approached Casablanca - which, despite air raid sirens going off at various times, was not in fact targeted by long-range German bombers from Tunisia. For all their vaunted efficiency, it seemed the Germans had no idea the President was planning to meet Churchill there, let alone intending to stay almost two weeks - Goebbels recording, afterward, his near-disbelief that the Sicherheitsdienst of the great Third Reich had actually intercepted enemy phone calls, yet had taken the name Casablanca to be Casa Blanca, or White House, Washington, D.C. (page 65-66)
Goebbels was thus floored by the seemingly authentic reports that finally reached Berlin on January 28, 1943. “The sensational event of the day, is the news that Churchill and Roosevelt have met in Casablanca,” the Reichminister dictated in his diary. He made no effort to conceal his amazement. “So the discussions have not, as we assumed, been taking place in Washington but on the hot coals of Africa. Once again our intelligence services have completely failed - unable even to identify the place where the talks were taking place,” he fulminated. “They’ve been held now for almost a fortnight, and they’re being heralded by the enemy as the gateway to victory.” (page 130-131)
Hamilton cites Roosevelt and Churchill: Men of Secrets by David Stafford for the first passage and Die Tagebucher von Joseph Goebbels (The diaries of Joseph Goebbels) for the second. (Hamilton states it is his own translation from German to English.)
The Snopes article cites The Casablanca Companion: The Movie Classic and Its Place in History by Richard E Osborne.
I actually have read Osbourne’s book but I don’t have a copy handy to check the text. But overall, I feel that Goebbels would be a solid primary source and I would tend to trust Stafford and Hamilton, who were writing histories of the conference, over Osbourne, who was writing a history of the movie.