How does a group of flies get into a house when the doors/windows are all closed? Do they have nests?

In the past couple of days, we've noticed at least 5-6 flies in our house, which we've killed right away. But for some reason, 2-3 reappear. Today when I arrived home after work, I saw three on the sliding door and went to kill them with Raid. When I started to spray, at least 7-8 flew out from behind the blinds. I was freaked out because all the windows and doors were closed all day. How is that possible? Please help!!!