Finding Notes on Mac
Outline is equipped with a fast and convenient search tool.
How to Find Notes
You'll find the search field on the right side of the toolbar of your Outline.

Enter an item you want to find and look for it in the listed pages. On the left side of the search list you will see notebooks that contain those pages.
Outline highlights all search results on the current page for your convenience.

Besides, if you're searching for something you worked on lately, take a look at the list of Recent pages in the upper-left corner of Outline window.

Finding Notes in Password Protected Section
To be able to search through password protected sections as well, you need to unlock them first.