Does the dash light (car with a key) blink all the time the vehicle is off weather or not the doors are locked/
If that's all it takes to flatten the 12 V battery, it needs replacing. It shouldn't flash unless the car is locked and the imobbilzer is enabled (which is what that flashing symbol means). The only (normal) way to lock the car without enabling the imobilizer is to manually lock it with the mechanical key.