Where does OneNote for Mac store its notebook files?
OneNote stores things in an application container in your user's Library folder. Open Finder and use the go to folder function and paste 

~/Library/Containers/com.microsoft.onenote.mac/Data/Library/Application Support/Microsoft User Data/OneNote/


The current version has an offline data file in a 15.0 subfolder. You can snoop on the program when it's running to see what files and ports are open.

sudo lsof | grep OneNote | grep -v Applications