It wasn’t as though he had looked, but scanner types like him that specialized in research had exceptional observational skills and intuition. It wasn’t time for the regular report from the human board on the moon to come, and she surely wouldn’t have engaged many Machines just to open a personal message. He had probably taken those factors into account, and instantaneously arrived at the conclusion that it was mail from Command.
You hardly need expert observation skills to figure that out.
