Have a look in ~Library/Containers/com.microsoft.onenote.mac/Data/Library/Application Support/Microsoft User Data/OneNote/15.0/OneNoteOfflineCache_Files
Any sandboxed app has its data under ~/Library/Containers/<app>, then from that usually under the Data/Library folder, which is a kind of mirror of your user library folder, just holding that stuff the sandboxed app is allowed to see.
Any sandboxed app has its data under ~/Library/Containers/<app>, then from that usually under the Data/Library folder, which is a kind of mirror of your user library folder, just holding that stuff the sandboxed app is allowed to see.