I have been trying to 100% anything I can until the latest update drops, and one of these is getting the Coin Case. I didn't really bother much with it back then, but now I have progressed so much that the sewers have been replaced by the Water Treatment Facility. I scoured the entirety of the thing and can't find it anywhere (yes I did talk to the woman who then flushed it).
I tried to look for answers, but it's either on an archived forum (so I can't ask for further clarifications) or of older versions of the game