Not sure if this merits a theory yet, but with the Dharma countdown timer hieroglyphs relating to the underworld, and all the apparitions of the dead, perhaps the door is meant as a passageway for spirits.

"A false door is a common architectural element in the tombs of Ancient Egypt. The ancient Egyptians believed that the false door was a threshold between the world of the living and the dead, and through which a deity or the spirit of the deceased could enter and exit." [1]Amyshmamy 18:43, February 5, 2010 (UTC)