I forget the chapter, but in the war-torn city early in the game (the one where you run from the jeep in the opening), there is a little side path in the central street of the city. After entering this street, there'll be a barricade of cars. Once you do some climbing and are on the other side of the barricade (which you need to do for the story anyway), stand with your back to the barricade. Now, walk down the street, and look for a small alley to your left, where you'll see an open manhole cover. Drop down into the sewers and you'll find the strange relic sitting there.