When you close a note in the OS X 10.9 Notes app it leaves no trace, there's no warning and no revert. It's just appears to be gone. Very unMac like to me. It's in user interface design 101 that if there's a risk of data loss, the application should inform the user of this is what will happen.
Where do the notes go in OS X if the default is "on this Mac"? What if the default account is an IMAP account? Can you get it back from the place it's connected to?