I didn't find it. Cyan told me about it. I thought I should link to it, considering I'm not active any more. This way, more people can enjoy the secret forums.
I have no idea how he found it. He never told me and he's not active any more (I think).
However, do you see those numbers in the URL? For this thread it's 8087497. I reckon he found some numbers that were invalid for some reason (creating empty threads with no linkbacks) and linked a bunch of them together to create the hidden forums.
Looks like you can just take random game comments and profile comments and turn them into threads that'll only be seen if you have the number of it somewhere. There was a thread explaining how to turn comments into threads, but I can't find it.