To search your entire library:

Click the search field on the lower right of the Things window.

Start typing characters. The results will be immediately shown in the main window.
You can search your entire library for to-dos or projects that contain specific text in either their title, notes, or tags.
Click the search field on the lower right of the Things window.
Start typing characters. The results will be immediately shown in the main window.
In the search results, headings indicate the list where the to-do or project resides. If you want to go to that list, you can select the to-do or project and click on the "Show in list" toolbar item.