Flies from nowhere, in my lounge

I came home the other night to ~20 big fat flies on the inside of my lounge window. Killed them but of course looked for source.

No dead animals brought in by cats, no food or drink around, no flies anywhere else in house. Hardwood floor, curtains, basic furniture sitch. No evident smell.

Next night - less flies but still flies, same area. Kill them. Stare like a hunted dog everywhere. Somehow, always a new fly.

Wfh today and all is quiet, no evident flies, no smell, then ...flies! I am going absolutely spare. Where are they coming from?? I can see two right now, and I really had gone on a very sharpshooter hunting type killing spree yesterday.

It's fall here & search says flies may seek warmth, but surely not this many, and they're the fat flies that scream dead carcass or garbage nearby to me. There's no place for anything to be developing sight unseen, and no flies on or in the rubbish bin/kitchen.

3 now. FML. Any ideas?