The "derivatives" folder under "Photo Library" showing hidden photos

If you want images securely hidden away then create a new library, put the images you want hidden into it and put that on a password protected Disk Image.

That's the only way.

Well, first, "Don't go mucking around inside the Photos Library package! You could damage the database!"

That being said, I tried looking for "hidden" pictures in a small test library I play with, and I found, in fact, 3 copies of a picture that I had put into the "Hidden Folder," including the original HEIC version. There may be more-- searching around in there is like being in those movies (like THEM! and The Third Man) where you're slushing around in the city sewers, nervous about what might jump out at you!

I, too, found this a bit disconcerting. The "Hidden Folder," before Ventura, was always open to anyone who just clicked on "Show Hidden Folder." It's never been very hidden. With Ventura you have to have a password to open the folder in Photos, but clearly you can get around that.

But Apple has never meant for the "Hidden Folder" to be especially secure. They've just added a little bit of resistance to casual peepers. I guess the moral of the story is, "Don't hide the combination to your safe in the Hidden Folder." Not a good place to store your passwords, either. It seems like they should have a little warning about this someplace so people don't get a false sense of security.

You can let Apple know what you think at:

Feedback - Photos - Apple

Maybe they'll add a warning. Maybe not.

Of course you could. They would just be in a different library and you can choose which library to open simply by holding the Option key when launching the app.

I left feedback suggesting that "packages" like the Photos Library packages require a password just as the Hidden Folder does. That would give the same level of security, which seems reasonable.

Yer_Man's suggestion of an encrypted disk image would provide much greater security, of course, and bypasses the idea of a "Hidden Album" completely.