This is a closer look at the collar where the flex duct had fallen off. This is the return duct. The return duct pulls air in as the supply duct pushes it out. Since this is pulling air in without a duct attached to it it is pulling in dirt and cobwebs from the crawl space along with any hidden allergens or spores in the air. Therefore, the homeowner and any other occupants of this residency are breathing in unhealthy air.