You can add any field to your form, click on it, and in its properties, enable the "Admin-only" option. This way, that field will be hidden on your form and can only be filled by the admins of that form.
To fill in the admin-only fields, in your Formaloo dashboard click on your form, go to the responses tab, and edit the response to which you want to add a note. Add your note and save. Your note will be saved and only you and other admins of the form will be able to see it.
Everyone who can view the form responses will be able to see the admin notes too. But only people with edit access can edit the admin-only field's content.