There are several related approaches you can use to limit access to this field. The first and most secure option is to put this field in its own related record so that you can use record level access control to limit access to these notes on a record by record basis.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.
On a more modest and less secure level, you can give your users different accounts with privilege sets that identify their level of access. A user with an account that has a "low level" privilege set is not allowed to see the contents of thes field. Users with [Full Access} or a high level privlege set are allowed. You then uses scripts to control the visibility of this field. The simplest to set up is to have two nearly identical layouts--one with the field on it and one without. When a user with limited access opens the file, scripts take the user only to the layout without the field. Users with higher access permissions are taken to the field that does have the field.
It's also possible to use a tab control or other means to hide the field from view and thus use just one layout, but the field is only visible when scripts/calculations determine that the current user is permitted to see that field.
Here's a demo file that illustrates several different ways to make objects appear and disappear on a layout: https://www.dropbox.com/s/8levaz6deiyzjr8/LayoutObjectVisibilityControlDemo.fp7