Houseflies appearing every day?

I’m not sure if this counts as an infestation, but nearly every day I see a new housefly in my house. I kill one, then within a day or two, another appears. This has been going on for a few weeks.

They wouldn’t be coming in through any open door or window, so I’m assuming they’re spawning in the house. Would that occur at such a staggered rate though? They typically show up in the same corner of the house on a south-facing window with sunshine.

My house was fumigated a few months back. I took a large houseplant outside during that time and brought it back in after. This houseplant is located in the corner that they seem to frequent, so I wonder if they’re coming from somewhere on that. Otherwise somewhere under furniture or carpeting in that corner.

Any info, suggestions, or what I can look for would be appreciated.