Welcome to the forum.
It sounds like there's a deficit in store vs. drain. If you have access to a multimeter and know how to use it, you can check for the amount of current the vehicle uses while sitting around and then start pulling fuses to see which circuit has the heaviest draw. That said, I have a feeling that the deficit is from the usage pattern (infrequent, short trips). The factory recommends that the vehicles in the dealership lots get started and idled for at least 30 minutes once a month to maintain the battery.
I've been using battery minders for years to keep my batteries topped off. The one I use now is a CTEK Multi US 3300 battery charger/maintainer. In fact, the first thing I did soon as I got my Malibu was to purchase an extra CTEK eyelet connector and install it. You may want to consider getting a battery minder (BatteryMINDer, Battery Tender, or CTEK brand). You can expect to get more life out of the battery to boot.
I've been using battery minders for years to keep my batteries topped off. The one I use now is a CTEK Multi US 3300 battery charger/maintainer. In fact, the first thing I did soon as I got my Malibu was to purchase an extra CTEK eyelet connector and install it. You may want to consider getting a battery minder (BatteryMINDer, Battery Tender, or CTEK brand). You can expect to get more life out of the battery to boot.