I have had flies appearing in my apartment. Where are they coming from?All doors and windows are shut.?

They are just hatched. They are coming from larva left by the previous flies.

Aside from the above possibilities (quite true--they DO lay eggs on other dead flies, like in window sills), it could be a source of food you can't see, like crumbs from eating on the couch or in bed (any slobs around?). We had this and there were larvae under the couch cushions, where all the crumbs had slipped down! So check those places too. They also LOVE places where babies + toddlers drop food, like car seats and high chairs.

Yuk. I had this happen to me once too. All these disgusting flies coming from "nowhere". I think they were hatching from somewhere. I had a new housemate at the time who had a box sitting in the lounge room. I decided they came from something she had in there. Lasted a few days and then no more. So gross.

Flies can get in around the windows or through a bathroom vent.

They could be coming through a vent maybe.