Home Dr

Well I'm not sure about all that high tech stuff. My 2 cents worth. It looks to me like leaky duct work in the attic causing a low pressure condition in the home interior. I test ducts leaking 200-400 cfm all the time. That along with poor sheetrock cutting that leaves a gap between the register and the ceiling/wall provides a perfect makeup air path. The air is very dirty after passing thru loose fill insulation in the attic. Notice the stain appears to be emanating from under the grill.