I apologize if this has been mentioned before.

When playing the abandoned subway level, you will run across a posted subway map from time to time. If anyone lives in Maryland, Washington D.C. or northern Virginia, you should immediately recognize this map! It is exactly the same metro map as is actually used around here. Thought that was interesting. I added pics.

It's not uncommon for environment artists to use things from real life to turn into art assets. Textures, props, heck even the skybox in Serious Sam HD is a panoramic photo taken from the side of the developer's studio (that's why if you look hard near the center of it, you can see a factory in the distance). It saves time and money when making the assets, instead of making things from scratch.

Good eye!