Go to the level Metropolis and get to the chapter

"This town ain't big enough for the two of us". Go to where all the Wraiths are and take the second left. Go to the end and you will find a sign that has a yellow stripe on it. Read it, it's entertaining.{I won't spoil it for you}. At the bottom it will say "Paul Humphrey Russell". Paul is one of the Single-Player Environment Artists. You can also blow up the sign.