Yep, been waiting for this one. The OneNote team are being a little coy about this one, either because they are super modest or don’t want to make a big splash until they’ve worked out some bugs.
Check it out. You can now search OneNote notebooks (at least today in iOS) even when they are closed. I’ve shared an example below, from a NoteBook I made solely for the purpose of renovating our master bathroom. We replaced a pocket door (blech) with a real door. That notebook has been closed for some time as I don’t need the info on my device but it’s super awesome to know I can find it later.