This video shows you were to go in the Waterways, level B1 if it doesn't pop for you during regular game play. I queued it up to the 0:45 mark because if you're here, you don't need to see how it "should" have popped.
Off the elevator, take the main path around to the right, and then turn right to where you get one of the billboards. I would assume you can get it during your playthrough, but I had to come back for it.