Weird one definitely.

If it were me, I'd clip on and measure the 0.5A, then start pulling every fuse in the fuse blocks one by one (under hood and under dash) and see which one causes it to drop.

Also, not quite related: I think this car is supposed to have a little hose connected to the battery to vent gases, but I don't see it on this one. It'd be good to fix that if it has it.

we did that. When we removed the on star inline fuse the draw went to.1a from .5a.

Ah, well there you go.

Now, leave OnStar'* fuse out and wait three days and see if the issue returns.

Now we've accounted for 36 amp-hours over three days, 28.8 of which are OnStar. Eyeballing here, a car battery in good condition should be able to do 100 amp-hours or more. I have a suspicion that something intermittent is happening at times when you are not testing. I'm thinking of like how a lot of first generation Dodge Durangos would leak rainwater into the liftgate, and this moisture, somewhere in the night, would cause the lock actuator to run continuously until the battery is dead. If you didn't happen to be there for the event, you never knew that this was happening, you just know that you walked out to your Durango in the morning and the battery is completely dead. Maybe something like this is happening with this car, not necessarily with the liftgate, but something.

I would make sure that you can lock it, wait 30 seconds, then verify no dome lights (or other lights) are on.

Maybe next time it is going to sit for three days, take a specific batch of fuses out and see how it comes out 72 hours later. If it lives, then something you unfused is the culprit. If not, then something still-connected is the problem. Rinse and repeat until you narrow it down.

My friend took his car to the dealer, it was a problem with the onstar module. They want $750 to replace it. Seems high to me.

Ouch. If he were to do that, he should make sure they are sure a replacement will fix it. Preferably in writing.

You could probably knock a couple hundred off by finding a good junkyard unit, but no guarantees as the dealer will have to program it to work with this car.

If your friend doesn't use OnStar, unfuse it and call it a day.

I had intermittent dead battery issues with a new battery and just a 46 milliamp draw. I removed the OnStar unit and battery was at 12.4 volts this morning in 20 degree weather where as prior to removing the Onstar module it would be dead by morning and need a jump.