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Please try the "Hidden box" widget in the Form Builder, and see if that works to your satisfaction.
You can read here about how to add widgets to your form:
Please let us know if you have any problems along the way.
What bokid said is correct, this would give you a hidden field, that would still show you the field in the Google Spreadsheet for example allowing you to add any extra details to it.
You could do that with some other fields as well and select Hide field option (you must right click on the field or click on the little gear wheel in the quick actions bar) so that they are not shown.
If you wish however to be able to edit the form from within the Submissions Panel and be able to add your notes to it, then you would need to approach this differently, for example as shown here: How to have multiple step Approval Form with Hidden fields
Do let us know if you have any further questions and we would be happy to assist with them :)