Its a dry heat right.

Seriously, Vegas has rain like every where else and I'm sure they had some roof leaks from time to time, so if the moisture doesn't come from say a leak in a duct it could come from the roof or a water line in the attic space.

As you say, it does just appear to look like some type of dust to me too.

What happens in Vegas, stays in Vegas.

So they can keep it whatever it is.

rick