Notes.app stores notes in an SQlite3 database located in
/Users/$USERNAME/Library/Containers/com.apple.Notes/Data/Library/Notes/NotesV2.storedata
and, if enabled, in an iCloud IMAP folder for sync between devices. You can view the contents of this file using the 'sqlite3' command line tool or, e.g., "SQlite Professional" from the App Store.