Help reaching S1 chest

[giothewatcher]

I tried, really.
I know I'm supposed to find a hidden door in the warehouse sector, but I can't find it. I tried to roam every room in search of a vague hint or a hidden interaction.
To help anyone helping me:
-I'm at the ending boss, the room right before it;
-I completed all quests, even the flower one;
-I have a sewer key in my inventory but I can't remember if I found a place to use it.

[giothewatcher]

I found it, in the end.
It's in C9 on a whiteboard.