You won't see anything on the audit history for the parent record (Case in your scenario).
Notes are separate records. You are not changing the Case by adding Notes to it.
You would need to look for audit information about the Notes themselves, or use some kind of automation to capture this. For example you could add a field to Case for "Text of latest note" and then use Workflow triggered on creation of a Note,
check if the parent record is a Case, if it is, copy the body of the note to this new field (just overwrite it every time). This will then show up in the audit history, but it is of course a duplication of the data in the Notes themselves.
Hope this helps.
Adam Vero, Microsoft Certified Trainer | Microsoft Community Contributor 2011
UK CRM Guru Blog
