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There is also a constant outflow of insects from the room which is less than the influx but not 0, there is a limited supply of flies outdoors to see and then also make it inside the room. Flies are not, however, a constant pressure, their numbers correlate to food sources in the area, as well as cyclic trends which will influence their numbers (predator behavior, etc.).

Regarding territory, sort of: "In urban areas, where people live in close proximity and there is garbage, flies have small territories and may only fly 1,000 meters or so. Rural house flies roam far in search of manure, covering up to 7 miles over time." Via: https://www.thoughtco.com/fascinating-facts-about-house-flie...