So... we're used to having fruit flies, which are easily trapped with a jar of vinegar or whatever.

But lately, with the warm weather, we've been seeing a TON of houseflies that seem to appear out of nowhere INSIDE our apartment.

Usually it happens like this: I see one or two, swat or spray them, then suddenly, I see like 4 more buzzing around.

I CAN'T FIGURE OUT WHERE THEY ARE COMING FROM! All the windows have screens, and while a fly might sneak in once in a while, I'm pretty sure this amount is coming from somewhere inside the house... My theories:

They are coming from the drains? But I'm not sure. Even when I was literally washing the dishes, they seemed to appear from elsewhere.

Houseplants? This may be it. Some of the houseplants used to live outside. So maybe the flies had eggs in there, and with the warm weather they are hatching?

They have a secret nest somewhere in the kitchen...

Anyway, out on the sidewalk, in the neighborhood, there are lots of flies. That's whatever. But it's super gross to have this many INSIDE MY PLACE. Any help, advice or commiseration is appreciated.

edit: Oh yeah, one weird thing/tip. At night, they all seem to go to sleep on the window screen. Last night I was able to just trap a bunch in a container and release them outside...