I would have been your daddy. There's a set of triggers on the Outskirts level, all of which you need to enable, involves a lot of roof-jumping and grenades. With a bit of luck, you'll get a skull from a certain roof and it ports you to an enclosed space, where you need to fight 28 elites, on Legendary. After killing all of those, the game supposedly unlocks a new set of comments for all the buddy marines and even Cortana.
Farfetched? You bet, but I've seen the skull and the 28 elites.