Battery dies

I have a 2001 Pontiac Montana. During the last few months I have had to replace the battery two times. If the car sits for a day it will not start and the battery is drained. I have had it in the shop for a week and they cannot figure out what the problem is. They have checked the alternator, cleaned the cable connections etc. and still no luck. The battery can be charged but if it sits for a day it is completely dead. Sometimes the car will start but the automatic door will not work. Also the outside mirrors change positions without anyone touching them. I would appreciate any suggestions on what this problem might be.