I think I'm the first person to find the secret room? I'm honestly not sure since I got it on my first playthrough and it honestly wasn't that hard to find. I don't know how to mark something as a spoiler so I'm going to put the image of the room in the reply to this comment.
Viewing post in Cursed Travels: Below the Factory comments
Not only have you been everywhere, but can promise you that you've been mere steps from the start of the path to the secret room, and just didn't know it, even before you started thoroughly exploring.
There's a pit you fall down to reach a room with gas tanks on your way to the end of the game. The secret room is on the other side of it. It's too big to jump across.
You have been on the other side of that pit already, you just couldn't do anything. You just need to get back on that side of the pit.
You need to complete every seal in the main factory as indicated by the office computers in order to open the path to get to the other side of the pit.
Then you need to go somewhere that you probably didn't think was an option, and completely forgot about by the time you finished all the seals.