How often should a car be driven to prevent the battery from draining completely.

I live in the city. So I unfortunately don't have many excuses to drive my car. It's a 2015 Focus ST it will matter. I suppose we drive relatively infrequently. At a minimum it's once a week but usually a few times a week. These trips are usually only a few miles to the grocery story. At least once a month we drive out to the suburbs to see family.

Last week I discovered the battery was dead. Like very dead. I couldn't even unlock the doors. Autozone charged the battery and tested it good. The car is at the dealership now and they want me to believe I don't drive it enough. This very well could be the case. The Internet says a car should be able to be left for weeks without being driven. I had driven the car only 2 days before and the drive was a bit more substantial. 30 minutes on the highway one way and again on the way back. Then 2 days later the battery is sitting at 3V.

So do I not drive it enough? Or is there a problem because I don't want the car to leave the dealership with an issue and I'm dealing with this again.