The answer to 'why is my air duct pet stains' is that pet dander, hair, and urine can accumulate in air ducts over time, leading to visible stains and odors. Pets that spend a lot of time in the home can shed fur and skin cells that get pulled into the ductwork, and any accidents or spills can seep into the ducts as well.
The stains and odors are a sign that the air ducts need to be cleaned to remove the pet-related buildup. This is an important maintenance task to improve indoor air quality and prevent the spread of allergens and bacteria throughout the home.