Battery dead in 4 days in my E30

So the E30 has a 2 year old Kirkland battery. I had to fly out a week ago and thought to take my car and let it sit. Well fortunately I hadn't driven my car for a few days and decided to check the battery. Well it was dead. So dead I couldn't even get dash lights.

I ran a 6 amp charger on it and I was able to start but it was dead the next morning. I had to take that flight so when I returned I saw that the battery only measured 5volts. Charged it again and went to Oreilly's to get it checked out. Starter and alternator tested good. Battery tested "marginally good" so i'm sure there's a weak cell. I guess it's time for a new one. I'm going to see if Costco will return what I have even though I'm not the original owner.