Causes of flies coming in during fall when doors have not been left open?
There may be several possibilities. The flies may be cluster flies or other attic flies which enter homes to overwinter. Inside swat or vacuum up flies. Prevent entry by caulking, sealing up openings, repairing screens, etc. See our website for more information https://extension.umd.edu/hgic/cluster-flies
Also, make sure there are no dead animals in your chimney, wall voids, along the outside of your house, pet waste, garbage, etc. There are many types of flies that can be attracted to this. All you can do is eliminate their breeding site and clean up these areas. See our website for more information https://extension.umd.edu/learn/flies