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This is of cause only my interpretation, but I also had the feeling that Mycroft was setting Sherlock up to go after CAM. First he brings his laptop with him (“Well, you shouldn’t leave it lying around if it’s so important.”), then he practically tells Sherlock that he knows what he is up to (“Maybe there was something in the punch.”) and of cause there is CAM’s comment (“Mycroft has been looking for this opportunity for a long time. He’ll be a very, very proud big brother.”).