The radio or ‘MMI interface’ should have an on/off switch. If the on/off switch is not working then the system or the radio is faulty and is permanently 'open circuit' in which state it could be draining the battery while the car is left standing. There might be two car handbooks: one for the car and one for the MMI/radio, and, having found the switch, you can test it yourself.

If the radio stays on whatever you do then demand that the Audi dealer either repairs it or replaces it under the new car warranty that he sold you with the car. If he refuses, write to him by post office special delivery (so it becomes a matter of record), outlining the events and telling him that you will now have the car repaired independently and will sue him for the cost via www.moneyclaim.gov.uk. Your rights are here: www.honestjohn.co.uk/faq/consumer-rights