What can explain flies in the house?

Answer 1

They have a food source. This can be anything, from exposed fruit to the trash can. Especially fruit flies can also sneak in at any time, most likely inside of fruit - eggs and larvae are very small and really unnoticable, I never once saw them, but the prescence of flies proves they had to have been there. Keep grapes and strawberries and such in the fridge so their development is slowed and the eggs don't hatch, and fruit flies will disappear.

Other flies may come in from outside, but once again, if they have a food source they'll stick around.