Why did my battery die after 3 days of not using the car?

It s a 2006 Ford Explorer 4.0 v6 engine.
I drove it to auto zone after jump starting it and leaving it on for about 20 minutes and the guy in the store said my alternator is good and my battery is good. I have a second battery in the back that I haven t changed in about 4 years but it s not puffy or leaking it s a kinetic 800 hc can it be that it s draining my battery while it s off

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