The disguised merchantmen that the Germans used were pretty successful I think, I doubt anyone thought that a European built freighter in the Indian Ocean would actually be a German weapons platform.
Still it's policy now, even in peace time, that when an RAN ship rendezvous with a suspicious vessel, the boatswains are on deck manning their weapons and range will only be closed after the vessel is certified to be safe (often through boarding it). Why Burnett didn't even have most of the weapons manned and closed range before investigating the Kormoran no one will ever know, in the end it cost over 600 sailors their lives.
