A section of the app that, when set up, has no obvious signs that there is a hidden section of the notes app. The only way to enter it (after you set the password for it in the settings) is to long-press a button/icon or something of that nature to initiate a password dialogue. For example, pressing and holding the hamburger menu. Then you have access to your hidden notes page which looks nearly identical to the home page.

It should work the same but automatically close/lock if you close the app. And no settings outside of the hidden notes page points to the fact that you have one set up. (That would kinda defeat the purpose! Lol)