I would start with the alternator.  Should be putting out about 14 volts with engine running.  If not, the car is pulling battery current without charging.  

can you please elaborate on "dies". So you turn the key and nothing happens or .... ? How long does it sit before it "dies"?

Are the battery terminals clean and tight?

The mechanic didn't have any wisdom to add?

Did you load test the alternator?

Well the tuff thing about parasitic drain is it doesn't always have to be present. But obviously their is a current drain or the battery wouldn't be failing as described ( and assuming it gets properly charged by alt and if your mech did the para test, I doubt he wouldn't have already checked the alt).

Easy bs example.

You leave glove box open (light) and drains battery overnight.  Well this only happens when ya leave the glove box open.  Now consider it has an intermittent/bad glove box switch (I'm not saying this is the problem although it could be, as an example).

Now other components can fail and act like this also.  First place I'd tackle is usb adaptors/acc plugged into your cig lighter... cuse me 2020, power outlet.

Other than that, learn to do para test yourself and keep doing it till you catch it happening.

Check the alternator it sounds like it needs replacing.

 I turn the key and nothing happens all electronics dead .. Alternator load tested it passed . Battery test shows it has lot of life .Did parasitic drain test overnight . Drain was normal. Mechanic said could be a bad battery cell. 2010  G37x with 50K Miles.  Dies randomly . OEM  Battery only 1 year old .