C230 Sport Coupe battery trouble

I enjoy driving C Coupe. It's a handy sports car in the city. However, the battery of my car have died three times within 8 months (brand new).

According to the dealer, my car consumed more AMP (battery) when the door is unlocked! (after turn-off the key & wait 30minutes till all the stand-by device is shut down). When door was locked, minor usase of battery was detected. Is this the way is should be? Or something is wrong specifically with my car?

My car is parked at the attended parking where nobody lock the door (I have to ask the attendant to remove the key from the ignition slot). So, it is difficult to keep the door locked in the garage.

Please advise if my car is normal (or not), and what to do?

Thank you.

Sato