Dear Bahaa,
Good day! Thank you for posting your query in our community.
Misplaced section always save unsynced pages or sections automatically for you when you open the related notebooks without syncing them well.
If you intentionally delete a section and no longer need to sync or save changes to it, you can delete the misplaced section.
Here is an article for your reference: Resolve misplaced sections in OneNote - Microsoft Support.
When you open .one files outside a notebook in OneNote, they land in Open Sections. If the contents in Open Sections are useful, you can move them to the corresponding notebook or close them directly. When there are no pages in the Open Sections, it will automatically disappear.
Sincerely,
Jazlyn | Microsoft Community Moderator