Halo 4

Just a quick q: Why couldn't Cortana hear the Didact when he was speaking to MC in the 6th (??) mission?
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Go the Mickapeckalin

My guess is that the Librarian instilled MC with a some forerunner DNA which caused him to be mentally linked to the other forerunners (ie Didact in this situation). Would make sense I suppose.
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Personally I assumed that because the Librarian speeded up Chiefs evolution, he now has the ability of a telepath, if only to converse with Forerunners. Didact was thus speaking to chief telepathically not using any technology which Cortana could hack into and eavesdrop on.