Hint that Christian is the smoke monster?
I was rewatching the season 4 episode, “Something Nice Back Home”. My theory comes from the scene about halfway through the episode, where in Jack’s flashforward, there is a smoke detector going off in the lobby of the hospital. As Jack takes out the battery to turn the detector off, Christian appears. This seemed like a very obvious clue to me that has been overlooked. A smoke detector goes off, and then Christian shows up.