The sewers can be found on the far left of the map. You'll need a crowbar to open it but once opened it will stay open unless you restart your entire game file. The crowbar is located in the woman's washroom in the Movie Theater Hub in the red tool box.
In the sewers you'll find a valve that causes sewage to fall from a green dripping pipe on the ceiling and many skinned bodies hanging by their feet from ropes. If you walk far enough into the sewer the slow-walking clown masked killer will begin to walk toward any character alive. The Sewer Killer, while walking slow, can poof closer to the playable characters so be careful.