It's outlandish that the Kormoran didn't know the secret callsign of the ship they were imitating.
Please see the question in my title, and the emboldened sentence underneath. I quote Wikipedia.
During the exchanges and distress signal, Sydney positioned herself just off the raider's starboard beam on a parallel course, approximately 1,300 metres (4,300 ft) from Kormoran. The cruiser may or may not have been at action stations: the main guns and port torpedo launcher were trained on Kormoran and her Walrus scout plane had been readied for launch, prompting Detmers to prepare to engage Sydney, but her 4-inch (100 mm) guns were unmanned, and personnel were standing on the upper deck. During her manoeuvre, Sydney appeared to signal "IK" (the short-form for "You should prepare for a cyclone, hurricane, or typhoon"), which Kormoran did not respond to, as from their perspective, such a signal did not make sense. The Germans were unaware that the letters were the interior of the real Straat Malakka's secret callsign, "IIKP": to verify her identity, the ship had to signal back the outer letters. The aircraft was shut down by 17:25, and the catapult swung into the storage position; the two ships were too close for a safe launch.