I read a study that your brain can respond to trained sounds when sleeping.
I slept through my roommate cutting wood with a chainsaw last week, but the moment my cell phone alarm went off, I was awake.
Fire alarms are supposed to wake you up, but at the same time, he could have awoken for a moment, figured it was entirely unimportant and tuned out the noise.
I woke up during a 5.4 earth quake, shut the door cause it was shaking and went back to sleep and have no recollection of the event because i discarded it as being unimportant.
I'm not completely surprised, but I also wouldn't be surprised if he had some previous damage as others suggested.