User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-75.119.225.228-20121006002039/@comment-3146930-20121006005301

The Doors of Death have two openings, one in the mortal world and one in the Underworld. The one entrance in the Underworld is in Tartarus, which is reserved for the most evil of beings (monsters, Titan's, mortals, ect). When Gaea took control of the doors and opened them, it allowed monsters a quick way to reform and mortals a way to escape. As far as finding their way to the US, monsters when they reform seem to just reform there, possible because that is where Western Civilization is the strongest, but it is never explained.

It is also shown in The Son of Neptune that with Thanatos gone, exits to the Underworld were popping up all over the place. I mean Nico managed to get Hazel out without using the Doors of Death in Tartarus, so there are other ways of exiting the Underworld. Thanatos is most likely focusing on the souls trying to leave Tartarus, allowing Echo and Narcissus to pass unnoticed.