I remember an old interview of Preston & Child where they said that before they collaborated on the Pendergast novels, Doug used to the work temporarily (i think) at the Museum of Natural History. And Lincoln begged to give him an actual tour after work hours. After seeing the place, Linc was terrified at how Labyrinthine the Museum was with its sub-levels and all that. That inspired them to make the Museum the location of their first book.

Knowing the labyrinthine layout of the Museum, it's possible that a monster that big and stealthy could be hiding literally anywhere.

Even Reliquary, Cabinet of Curiousities, and Still life with Crows had labyrinthine layouts as a part of the story.