Flies in the winter. Yes.

All of a sudden seeing flies in your home? They could be cluster flies. Cluster flies find protection in your home during September and October, looking for a crack to get into and a warm void to over winter in. When you get a warm day in the winter these flies come out. Sometimes they are just s few, sometimes there are many, but most times they are confusing and scary to the home owner. These flies can be controlled with spraying voids, and you can also try to cut them off at the pass by sealing holes and fixing screens outside before they come in, although this can be grueling and hard to get all the holes. I usually just recommend a vacuum with a hose attachment to suck them up as they become visible.