I've seen situations like this. It could be happening for a couple of reasons. First, does your home have a history of rodent issues at all? There's a type of fly that will all of a sudden invade for what seems like no reason, but they're really after something dead that could be hidden in the wall structure of your home. I'd definitely recommend taking a walk around the outside of the house, looking for any gaps or possible entrypoints, especially near cables or electrical lines or even your AC unit where they attach to the house. A mouse can pretty much flatten their bodies out to squeeze into the tiniest spots. Think of a mouse's body like your ear cartilege. You know how easy it is to bend or fold your ear over. That is how mice can get into houses! If you find any sort of crevice, get that expandable foam stuff from Home Depot or Menard's and seal those up to prevent any kind of critters from getting in before cold weather comes back. If you do end up discovering entrypoints, this will explain the flies. They're feeding on something decomposing. I know, it's so gross to think about, especially when you're a clean freak like me!
The second reason these flies could be clustering is simply the time of year. Depending on your geographic location, it could be because a nearby crop was just manicured, which sent flies flocking to residential areas to bask in the sun. Are these flies mostly on the south or west sides of the house? If so, a professional exterior power spray treatment will put an invisible shield on the outside of the house and make them drop dead as they land on it. That type of service may also include spot treatments around windows and doorways on the interior of the home, which will knock pests down for a solid three months or so. It will help with the flies, asian beetles, boxelder bugs, ants, earwigs, spiders, etc. Where are you located? If you're in the midwest, or near Chicagoland, we can help! :)
Best of luck to you!
Rose Pest Solutions