The note is a generic footer included with all emails so it's not a part of any specific message and its its own language string. Are you not able to find this string using the built in language finder shown below?
www.joomlapolis.com/blog/kyle/18712-language-key-and-text-finder
Part of your issue seams to be the text you have is not the default text and maybe is why it's hard to find. Either you've already overridden it or maybe have a bad translation in your language plugin. The default text is as follows.
Key: EMAIL_NOTE_AUTOMATIC_GENERATION
Text:
Code:
NOTE: This email was automatically generated from [sitename] ([siteurl]).