Some of the comments have mentioned that the Awoken may already know the true nature of the Traveller and remain hidden in the Reef. I also think that when Guardians are revived at the beginning by their ghosts they have their memories wiped (Exos, Awoken and humans alike). This would explain many of the grimoire cards mentioning Exos having visions of shattered memories whenever they die (kinda like Maze runner except with death rather than dreams). I think shattered memories would be a great way for bungie to introduce a personal storyline to the plot if they had plans to do so.