I had like 15 or so flies yesterday. Opened the window and let them all out... that was hard work, because they are dumb as a brick. Went to work and when I came back, there had been another 10 to 20 flies. Thing is, all windows were closed, so they must have come from inside. I've let them out again and then started looking for a source, but couldn't find anything at all. There was no old leftover food, nothing that would smell, or a ventilation or whatever... I really have no clue where they came from.
I'm hoping that I just missed something and they got inside over the other day when I had everything open for hours.....