Thanks, guys! These are all excellent ideas. I should add that the residue smells and tastes like exhaust. Even the service manager agrees that it is. I’m not sure about manufacturing residue. We first noticed the problem when we got discoloration and residue along the body just behind the rear doors. We chalked that up to road grime working its way up the space between the door and the body. We no longer see that. My wife also pulled some grittier stuff out of the holes in the hatchback door. Might have been some sort of build-up.
I will take all of your suggestions. I have a reliable mechanic I can get to conduct the ‘sniff test’. I don’t take the Hyundai to him because it’s still under warranty.
Thanks again. I’ll post updates as they occur