Open Notes.app notes in an external editor
If you search for some local notes with Spotlight or Alfred, the results will contain files like ~/Library/Containers/com.apple.Notes/Data/Library/CoreData/ExternalRecords/*/IMAPNote/_records/0/p8.notesexternalrecord. Opening them opens the notes in Notes.app, but they're just empty property lists.

The contents of local notes are stored in a binary format in ~/Library/Containers/com.apple.Notes/Data/Library/Notes/NotesV1.storedata.

I don't know if there are any other applications that can edit or import the notes, but there's no rtfd files for them or anything.