[danachevroletfor1967]

I wanted to post this and get some feedback from you guys. Is there an easy way to check if a battery is being drained even with nothing in the car on? I bought a new battery (AC Delco) from my local dealership in June, 2020. Last Dec., 2020, I asked them to check the battery as my car wouldn't start right away and gave all indications of a weak battery. They said I didn't drive the car enough to keep the battery charged. I have previously let the car sit for up to 2 weeks between drives with no problems.
I checked the battery with a tester and it showed low, 5 volts, after sitting for a week. It finally started the car after 3 tries. The alternator checks out fine. After driving for 20-30 minutes and shutting the engine off the battery shows 12.5 volts. If I let the car sit for 2-3 days and try to start it I get the same slow response time.
I'm guessing either something is now draining the battery that didn't before while the car is sitting in the garage or this new (from June, 2020) battery is a dud and can't hold a charge. What do you guys think?