You notice that your customers mention things that were only discussed internally in Private Notes. You would like to know why customers are able to read the Private Notes attached to their ticket.
This situation occurs when you have customized your Reply Template to use the
case.posts placeholder which copies all posts, public and private, in the email reply to your customer. To prevent this from happening Restore the Default Email Template Codes or Customize the Email Templates to change
case.public_posts which will only copy public posts into the reply. See the Using Placeholders to Insert the Latest Previous Public Post in Agent Replies article for details.