Son had a dead battery in his Mazda a month ago. Took it to Advance Auto. They put in a new battery and said their instruments showed his alternator was good. Did not drive the car this weekend and woke up to a completely dead battery. Wondering if the battery is bad or if there is something in the car draining the battery.

The Alternator may have been Intermittent, tested good, then Bad. If it is not Charging now, Most likely needs to be replaced.
3 Reasons for dead Batteries:

1 Bad Battery(water?)
2 Not Charging
3 A Draw

A Current Meter will show amount of Load or Draw on the Battery.

Remove Fuses 1 by 1 to see if you can locate any heavy loads. They have Clamp On DC Amp Meters, Are nice as you don't have to open the circuit and get between the Load. Good Luck

Battery was dead as a doornail. He jumped it off and drove it to Advance Auto. The alternator charged the battery and AA said the battery was good. It's a mystery so far.

What year and model is the Mazda? All cars have a slight drain when sitting but it should not drain the battery for many months. Did they test the old battery? Car might have a short somewhere.

2009 Mazda 3, at least I think that is what it is. Something must be draining the battery. It went from fully charged to dead as a door nail in one weekend.

You could have New battery tested again, Some new things are bad.
Alternator may be intermittent? Drain still a possibility.
I would get something like this, Cigarette Lighter Voltmeter:

http://www.gearbest.com/car-gadgets/...FQW1wAodEkIH-g

It will show if alternator is charging while you drive.

Finding a Draw requires removing of fuses, one at a time. unplugging connectors, Disconnecting Alternator, Ran into a Seatbelt Interlock that cause excessive draw when parked?

End of story. He must have left something on because the battery has been good since then.

I will say this, new batteries can go bad, we sell the wheel chair type batteries and some car batteries. We sell the MK brand. While rare, we have some bad in a week or two.

It seems like it is good. No problems since then.