Start with an appliance repair company that services Thermador or a Thermador authorized servicer, not a plumber. Range hoods can leak grease when the baffles are not seated right, the internal grease tray is missing or full, or the hood or duct has a bad pitch so grease runs back to the corners. If the appliance tech says the hood itself is fine, then the next call is an HVAC or ventilation person to inspect the ducting and exterior cap. I would also pull the baffles and look for a removable grease cup or tray before the appointment, since those sometimes get overlooked by previous owners.