So speaking of private fields, we musn't forget that we can also add fields to the Comment section of the node. In this case, I added an "Admin Notes" text field with edit and view permissions limited to the administrative user. Here, the admins can write notes to each other as log entries or notes to self. This field can also be used to close the ticket with the string "close ticket" and it will not be seen by the user.
Notice that we can use help text here to remind the administrative user how this field can be used.