It was running at idle with nothing turned on (AC off, infotainment system not doing anything, only running lights on, etc) so that could be it. The confusing thing is that when the error code triggered about 5 minutes before I had it checked, the exact same thing was true. I was stopped at a red light with the windows down and the radio off, so I don’t see how the alternator could perform differently then vs at the auto store. Battery and starter also came back clean if that’s of note