Maybe a model action that updates notes__c when a target field is updated with " {{notes__c}} 2000HZ:Passed" . This may bring in the existing notes then append it with the Appropriate phrase and insert it back into the not field. You would need a separate action for each target field and it wouldn’t be able to be dependent upon a specific answer. If it works, it won’t be a perfect solution, but it is probably your only easy way out. Otherwise, it sounds like a custom snippet to me.
