Battery weakness

#1
Hi , I have a toyota corolla since October 2014, and I have a problem I never had before ( even with another toyota corolla between 2008-2011): When I park for more than a few days ( one week is enough), the battery empties itself not to zero level, because, we can open the car without the key, light on the lights,etc...
When checked by parking service after two weeks standing, it revealed 8 out of 12/14 Volts ( can't remember).
I don't understand where it's coming from, and the garage keeps saying it's my fault, because they see it after I already ignited the motor with external help.
Anybody with an idea?
 
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