Flies inside the house!

Two friends each have a second home in the Southwestern section of fillewithdMaine. Both houses are filled with flies all year long. We can arrive in February after weeks of freezing cold, and when the house is warmed-up, dozens of flies come to llife. There is nothing dead or rotting that I could ever find at either house. The flies start out sluggish until the place is fully warmed up,but they always seem to be less agile than regular outside flies. Anybody know where these flies come from or hoe we can get rid of them. If we kill them all, there always seems to be more the next day.

Drain flys?