User blog comment:Darkcloud1111/''The Mark of Athena'' Plot Holes/@comment-3146930-20121014230006/@comment-8790199-20130427045807

Narcissus and Echo didn't escape through the Doors of Death. Neither did Gwen. What happened was that because the Doors of Death are open, the barrier between the living and the dead is weakening. The Doors of Death, whilst they are a physical passageway, are mostly symbolic, representing the barrier between living and death. This means that some dead become living, and some demigods/legacies/mortals/monsters can chose to go back to the land of the living.

Also, the Doors of Death were found through questing. Nico went on a quest to find them. Not officially, mind you, but he most certainly did.

I believe that when Thanatos said that the doors were not in a physical place he meant that their location shifts, perhaps faster than Olympus or other places. Think about it.