Dead battery - Is the alternator bad?

[3point2]

Hi-

I was driving one day on the freeway and the radio shut off and I couldn't get it to turn back on. However, the car was running fine. I pulled the car into my garage that day, armed the car, and came back 5 days later to find that my battery was completely dead. I took the battery to Autozone to have it checked out and they said it is fine.

My next thought was the alternator needs to be replaced. But I was thinking that since the battery completely drained after sitting in the garage for 5 days, it may be something else that's draining the battery. Or is this normal with a bad alternator? Could it be the voltage regulator or something else?

Thoughts?

Thanks for any help!

-Leo