Use deactivated form fields for internal information.

If you want to store information about your customers that should not be visible to your customers, you can set deactivated form fields to be used for internal information.

Normally, deactivated fields are hidden completely. If the use for internal information is activated, the deactivated form fields are only hidden from the customer. The admin has access to the fields everywhere.

You can activate this function in the admin area under Configuration » General settings » Application Settings » Internal Information.