Discovered this out of print book with an interesting topic. From amazon, Code-Words-Tunnel-John-Elvin/dp/B0006QW6VE (linking this site is banned? how you supposed to show a book?)

Story is, there are service tunnels all over DC, so you may never see any trafficking because it's underground.

Got this lead from IT JUST GOES DEEPER AND DEEPER -- Richard Sauder, where they bring up Pizzagate.