Q: Battery test after-market stereo question

I installed an after-market stereo in my car about a month ago. It works fine, powers down when I turn off the car, and disconnects from Bt connection to my phone which I think means that it is wired correctly to the power source. About a week and a half later I had problems with my battery dying. I started the car fine, drove about 10 minutes away and made a brief stop, and then the car wouldn't start. I got jumped and drove to Autozone. They tested the battery and it read out as being in good condition. Then it wouldn't start again. The AZ guy replaced my severely corroded/dirty battery terminals and then the car started fine. I haven't had any problems since. Today (2 weeks later) I went to Firestone to have new tires put on. In their readout I saw that they noted the battery is failing. I have two questions: 1. How can the battery be failing when Autozone recently said it's fine? 2. Could the stereo somehow be slowly draining the battery without outright killing it?

My car has 140000 miles.
My car has an automatic transmission.