Long time user of OneNote. I just noticed the new placement of the Search box that was previously on top of the Add Page area on the right side. At first it was very uncomfortable to use and I really do not like it there - covering up the middle instead of staying over on the right. Most ask for a option to put it back but this may be a programming difficulty.
I have found a work-around of sorts:
Click on the 2-headed diagonal arrow to the right end of the new search box location - Full page view.
I have 3 icons in the upper right corner:
Click to view other notebooks [ notebook picture with a down arrow ]
Show pen toolbars [ pen icon ]
Normal view [ 2-headed diagonal arrow ]
I click on the 1st one, view other notebooks.
Now on the first top line there is a SEARCH magnifying glass icon - click on it to get the old search box back!
May be a bit more clicking but some may find it useful.
Dave