I am confused. I have diagnosed alternator problems years ago, but it appears that things have changed and much of what I thought I knew no longer applies. I have a 20015 Buick Verano that seems to have a 350 mA draw from the alternator when car has been off for over an hour. When I test the alternator out of the car with ohms meter, I get a reading in both directions, I would have thought that meant that diode was bad and draining battery. any help would be appreciated.
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I haven't had much experience with 201st century cars. They've not given me much trouble yet.

. . . but anyways:

How are you testing this?

I am using an amp clamp on the positive wire coming from battery going to alternator.

What does voltage look like when the engine is running?

You have a parasitic draw with the alternator connected and it drops to zero when disconnected. If that is correct the alternator has insulation problems and needs to be replaced.