I think you can find out by paying for the members of Patleon.
Nah, I'm not that of an asshole to hide part of the game content behind the paywall.
If you see anything interesting, then you can interact with it and use/open/pick it up. If you can't do it, then I haven't finished working on it yet (but in this case I display a notification that this content is not yet available).
In the case of this box (on the roof in Red Light District), take a closer look at the walls of the building where you found it (or just check the previous page of this thread
)
As for the Secret Gallery, it doesn't have any content that is not available in the game for a regular player. That's a gallery of all noteworthy scenes the player has unlocked to view them again without loading a save file. This is just a bonus for my patrons, since as the small Patreon creator I don't really have many ways to thank those who decided to support my work. But if any of them want to share the code - well, it's a gift to them, they're free to do whatever they want with it (although I would, of course, prefer that they do not).