Little Hawk-

Your very point was made by the author of THE SECRET RAIDERS, pp. 228-229:

"As early as January of 1940 one of our own Q ships whose guns and torpedo armaments were about the same as the Kormoran's was intercepted off Sierra Leone by the Neptune, a sister ship to the Sydney, which was unaware of her true identity. The cruiser approached, and remained for some time steaming at slow speed, within a few hundred yards of the Q ship whose captain later reported to the Admiralty that, had he been a German, he 'could have disabled (the Neptune) with two torpedoes and swept her upper deck'. But such secrecy enveloped the work of the Q ships that the report was never circulated to the Naval Staff and the fate which the Neptune escaped actually overtook the Sydney more than eighteen months later."

Charley Noble