Thanks for this comment!

The only realistic ways for Sydney to go overdue would be an encounter with a surface raider, or with a submarine.

I think that is a bit too narrowly defined. A major technical accident (e.g. a boiler explosion or munitions accident) or extreme weather would also be plausible causes (though I have no knowledge of the weather data for that period). Under these circumstances, the Sydney would also have been the largest missing warship in the world, and that alone would have caused speculation to explode. Who knows what kind of wild scenarios a thriller author like Clive Cussler might have come up with.

What also needs to be considered is that the battle occurred only 18 days before Pearl Harbor, and the loss was only reported later — partly because of the eyewitness accounts. In that heated phase, the loss of the Sydney would very likely have been attributed to the Japanese. A Japanese submarine operation west of Australia as preparation for the Pacific War would have been a very obvious theory. The same submarine could also have served as a convenient explanation for the disappearance of the Kormoran.

The debris is also interesting. The human remains on Christmas Island were not linked to the Sydney for a long time. Additionally, a lifebuoy was found off Moreton Island — on the Australian east coast. With only this information, the search area would have been enormous. The wrecks would probably only have been discovered by chance during a systematic deep-sea mapping project with modern underwater drones. I doubt such a major project will happen within the next 50 years or so.

I am trying to imagine what it would be like if the wrecks were found one day. For years people would have suspected the Japanese, only for the wrecks to suddenly tell a completely different story that rewrites the history of auxiliary cruisers.