That's what I thought too, but how about this: You've just killed Crota, and you flee where you were (inside the moon during the Crota's End raid). If logic has any place to apply itself, by entering deep into the Hellmouth twice, by which exploring the one surface-accessible "base" of the Hive. Now, by going to the bottom of the Hellmouth, the unaccessible pit-of-death, and slaying the grand schemer, Crota, at least to our knowledge, we would have mapped out the Hive's entire presence on Earth's moon.