I tested this out with mine tonight, and after about an hour of listening, sure enough, there was some (pretty light) condensation along the edge of the metal earcup interior. It was about 65 degrees in the room, mid 30s outside and some humidity, but it was not noticeable inside. I did see there appears to be some sort of plastic-looking backing behind the metal holes in the earcup, maybe that’s to prevent moisture from getting into the driver? Still not good if it’s affecting other sensors, of course.
I really hope Apple responds to this soon, otherwise I‘m afraid I might need to return them since my January 8th deadline is almost here. I really don’t want to, I love these things. They’re the best sounding wireless headphones I’ve ever used and the AirPods simplicity is huge for me, but I don't want to have to deal with this affecting functionality or possibly damaging the headphones. And I paid the high price tag because I expected them to last for many years to come.