This was one of the raised questions for "Dark Cousin" and it is one of my personal questions as well. When Lana told Sister Mary Eunice that Dr. Thredson was Bloodyface, Eunice seemed almost surprised. She had to think back to the exorcism and then it all made sense. Numerous times throughout the season, the demon has boasted about the fact it knows everything. Yet it seems as if it did not know what Lana was talking about and seemed shocked. Interestingly enough, the demon did seem to know this information when it was posessing Jed.
Was the flashback played for effect as if to show the viewers the awesome foreshadowing? Or did Sister Mary Eunice really just realize about Dr. Thredson. If so, why? Especially since the demon showed the knowledge when it was occupying Jed.
This brings up the question about how much the demon actually knows. Is it really all knowing? Or does it have a limited scope of knowledge?