I talked to a new, more experienced vet about him today, and they said while he is really young for it to happen, it sounded like a heart attack. The surveillance didn't show anything out of the ordinary, except my youngest rat, skittle, kept going towards the bottom of the cage, looking toward where cookie was found, and running back to the top. He was doing that over and over, like he was kinda freaking out. But cookie was found with his legs stretched out in front and behind him, in a corner, on the ground by his space pod. His eyes were still open, but my son said he looked like he was just laying down, relaxed. They said he could have had a congenital defect we never knew about or picked up on? ¯\_(ツ)_/¯