just fyi, I have a 2005 BMW Z4 and it sits from Nov to March during the cold months (Northeast). I start the car once yes once a month for 15 mins and that's it. It sounds like you have a electrical draw somewhere in the car. Anything attached in the lighter socket that may be on when you park the car? If not find someone that knows electrical work to check the car out. Batterys don't go dead by themselves.
during the summer months I drive some times 10 miles a day and only a few days a week and still never had a problem. BMW needs to look at the car and determine whats wrong. No guess work....OH by the way my car is fully loaded auto roof etc. one other note if a power top make sure when you put the top up or down you have the motor running..