I followed that pig creature through the sewers. At the exit I found s control panel with a lever. When I pulled it, the water level behind a metal gate with a red diamond with golden dragons on it went down. I was able to find this door in the Former Disposal area but it is locked. How do I unlock it? It’s obvious there is something back there if the game let me trigger something behind the door. I looked around the area for a key and found nothing.
This comes later on as part of a quest. You may have noticed a few of these doors during your times in the sewers and found them all to be locked. You will get a quest further down the road to find all of these secret sewer stashes. You will get the key to unlock them as part of this. It’s a side quest so you will have to pick this up and follow it. The main story won’t push you forward to unlocking them