Battery discharging

For the last 6 months my well maintained 2011 Audi Q5 has had an intermittent problem with the battery discharging overnight, in all about six times.  The battery (only 2 years old) has been checked by two garages and both garages said it would be a problem with a computer not shutting down. It was at an independent Audi garage for 3 weeks but didn't fail in that time. It can be ok for three months and then fail twice in a week. There is nothing (such as a phone charger) plugged into the cigarette lighter. It was suggested by my Audi garage that the problem could become more frequent and would be easier to diagnose but that hasn't happened yet. Any ideas gratefully received.