To double down on obscuring the knowledge that the journal exists, if a person hasn't yet started a journal, a very good habit is just not to tell anyone you have one.

If anyone occasionally happens to see you writing in a journal and asks about it (though preferably it's best to save writing for private moments), you can just describe it as note-taking or brainstorming. If one considers these lies by omission, a more direct way would be a description of "personal notes." Then, the notes sound far less interesting for most people to look into.