1999 Jaguar XJ8 Battery

I have purchased a brand new battery for this vehicle and for some reason if the car has not been driven for over 2 to 4 days the battery is completely dead. I took the battery back to auto zone where it was purchased because it was under warranty and they supplied me with another battery. Went out of town and me car was in the garage for 5 days, and upon my return the battery was again completely dead. I had my car towed to the repair shop and they recharged the battery and ran a diagnostic check and could not find anything mechanically wrong with my vehicle. Do you have any suggestions.