What is wrong with my car!

One month ago, left it in the airport parking lot for 10 days. Came back, dead battery. Okay, maybe I left the lights on or something. AAA jumps it, it works fine for the next month.

Last week, left it at the airport parking lot for 6 days. Came back, dead battery. I know I didn’t leave anything on this time. AAA jumps it, it works fine for the rest of the night.

Next morning, dead battery. AAA jumps it, I take it to AutoZone. “Your battery is between ‘weak’ and ‘bad’. You should get a new one.” $100 for a new battery. Works fine for the rest of the week, including much driving between NY and MA.

This morning, dead battery. AAA jumps it, take it to a mechanic. “You need a new alternator”. $560. Pick it up, starts fine in their parking lot, drive it back home.

Tonight, dead battery. No sounds. No clicks. No anything.

So since it’s clearly NOT the alternator OR the battery, what is wrong with my car?! And who can I take it to that won’t screw me?!?!