Hide sensitive docs

There is some text that I don’t want potentially visible on the screen all the time.
It could be passwords, or more personal notes.

Could there be a feature for hiding sensitive information from appearing, unless you explicitly want it to? This should apply to search results, move-to list, etc as well.
Applying the hiding at the document level would be fine. At subtree level would also be nice, but probably more complex.
I’m not necessarily asking for super security (like pin or password), just a feature for making some content more discrete when other might be staring at the screen