Help with voltage regulator!

2010 Mazda 6
I currently have a problem with testing my battery and alternator. The test ran voltage regulator fail with a new battery and alternator. The car drives fine all week and if and when it sits over the weekend it won't start on Monday. I'm not sure where to even start looking to figure out A) what's draining the battery, B) why the voltage regulator failed the test and if that's the cause of the battery draining. I have a multimeter but don't know exactly where or how to start testing what's drawing power while the car is off. Right now after a day or so of not running the battery is reading 12.1 volts. While running after fully charging the battery the car sits at roughly 13.9 volts. Any help would be appreciated. I was told by the "experts" at autozone that the alternator was not the problem nor the battery as it passed the load test, that the ecu was the problem...csm anyone shine in on if a parasitic drain would cause a voltage regulator to fail??

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