What is wrong with my Volkswagen's battery?

Can locking your Volkswagon Passat cause the battery to run down? I travel a lot by plane and do not use my car for weeks. My friend takes care of my car when I'm away. He doesn't start the car everyday but does start it every week to keep the battery from dying. It was less than a week and the battery is completely dead. Is it due to locking the car? I notice a light blinks when the car is locked. Should I have him restart the car everyday so the battery won't die again?