Put a couple cups with: 1/4 to 1/2 filled with apple cider vinegar and afew drops of dish soap out they’ll go straight to it and it’ll trap them. Then find where they’re swarming to. Make some fly traps: Dollop of dish soap in the bottom of a cup or container. Fill halfway up with Apple cider vinegar. Then blast just a bit of hot water into the vinegar - just enough to make the soap on the bottom bubble up. Cover with plastic wrap and poke a bunch of tiny holes in it. The flies are attracted to the sweet vinegar, but get stuck in the film of soap on the top layer so that they drown. And they can’t get back out of the container with the plastic wrap over it, they’re not able to find where they came in.