score:5
I think there is a basic misunderstanding at play here:
It's outlandish that the Kormoran didn't know the secret callsign of the ship they were imitating.
No, to the contrary.
The Straat Malakka was not under German control at that time. It would not make much sense to "disguise" a German raider as a German merchantmen when trying to slip through Allied controlls! (The ship would be either captured or sunk, regardless. Merchantmen of warring nations are not neutral!)
The "free Dutch" ship had a secret callsign assigned by the Allies. (That is why the HMAS Sydney knew that callsign in the first place and could challenge the Kormoran this way.)
Naturally, that secret, Allied callsign was unknown to both the crew of the Kormoran and the Kriegsmarine in general.
Upvote:3
Such codes tended to be temporary and change on a regular basis, sometimes for each trip. Combined with the fact that as stated the code transmitted was normally to indicate something else, the crew of the Kormorant probably weren't aware of the reason they were being hailed with that sequence.
Had the German intelligence services known of the code and been able to notify the Kormorant, they'd probably have done so. They weren't of course infallible. In fact German intelligence services throughout the war had a rather poor track record overall.