I live on the 9th floor of a new 2-bedroom condominium, built in 2010, near Eglinton TTC station in Toronto. No plants. All food or produce lie in the refrigerator, and nothing is left in the open.
Now, and even in winter, I see around 1 fly that fly around in my unit at least once a week. But I can't figure out why! I've identified 5 possible sources:
Garbage can. I empty the garbage every day, and I don't see flies in the garbage can that I sanitize every week.
Balcony door and windows. I never open them, especially in winter!!! It's shut and locked. Anyways, all my windows have insect mesh screens.
Front Door. I asked my condo's superintendent if the flies are flying from outside thru the building`s corridor and public area ventilation, then sneaking past the gaps at the side of my front doors. But he said all corridor air must pass a MERV 12 filter.
Bathrooms ceiling fans? Perhaps the flies flew inside thru the bathroom ceiling fans, from the outside? But then why would there be flies outside in Toronto's frosty winters?
Kitchen Exhaust Fan? Same difficulty as 3.