Hello Greg,
I am sorry, but "Dropped by CASE" means that the message was dropped by the Anti Spam scanning feature (based on the Appliance config you are using).
The Appliance are not storing them with the drop and so, there is no possibility on the Appliance to report the message as false positive.
If you are not using the Spam Quarantine so far, you should check if it would make sense to do this for the spam messages instead of dropping them. You can also use the Quarantine without the need to allow the End-Users to access the quarantine.
Best regards,
Mark.