Under the bridge on the map Narrows, you start off on one of two sides, the red or blue side. Make your way up the middle of the bridge on the path ahead of you, the middle path, once in the center of the bridge look to either side of you and there will be two paths leading underneath where you just were standing, leading to a narrow connecting bridge which is clear.
Look to either side and you will see a part of the bridge that is not connected to the top, it seems far away. It's a straight piece that connects the two ramps that you just went down to get to the level below the top. If you crouch jump at the right time you can land on it. I have yet to see anyone go there except myself.
