I found the secret. However, I'm not proud of what I had to do in order to figure it out. So I won't reveal it, just in case someone finds it the intended way at some point in the future. It is indeed very well-hidden, though, so I don't really expect this to happen. I doubt I would've ever found it had I not cheated.
Using the same tricks, I also discovered another secret nobody seems to have found so far. (I'm not talking about the fish. That one was discovered long ago.) This one could be somewhat easier to find.