Both a fly larva or actual fly needs moisture and a food source to ever sustain life. A single fly lays about ~100 eggs in a day. In as little as a day they can hatch. The math here implies if there was one, there would be many!
Here's some fly info:
Now, while a fly will live for up to two weeks at best, I'm betting the one (1) you see in your apartment does not. If it did AND (a big and) it had a food source in your general area, you would be seeing many more than one. They would be around for quite a while!
Now you have also said you've seen these lowly single flies through the seasons, so while we cant rule it out, it's not likely one thing you yourself are doing and it is also probably not originating in your apartment. [for all the math reasons in my first paragraph].
This leaves us with the strong assertion it is coming from outside your apartment. This means it is getting in from an opening (one or more), to your apartment.
What are your openings?
- Door(s): Inspect the seals, if there are any gaps, address them
- Window(s): even with screens in place, flies can get past the framing sometimes. Not likely if the window is actually closed.
- Plumbing (think for example a dry trap, allowing them in from the vent), or just in the drain itself: Just run water sometimes. Are there drains in the apartment you rarely use?
- Air Return/HVAC: As noted for #2 above, filters are not often perfect fits. Especially for a fly, if they are in the walls/structure, a filter rarely fits a box they are housed in tightly on all 4 sides. Perhaps just inspect. Even just use a flashlight and see if there are any dead in the ductwork. Make sure the filter is well fitted when put into place.
Service conduit, electrical outlets, light fixtures, gas piping: This is probably the most likely cause.
- More so than any other. The cause can be # 6, 7, or 8 below, but you cannot control those. What you can do it check light fixtures, around kitchen appliances that are wired or plumbed, under sinks where pipes go through, wall outlets, the furnace ductwork entry into the apartment, any wall mounted appliances or devices that penetrate drywall or ceiling - yes this includes the fans, but as you said, its not as likely... so look at these other sources and strongly consider #6 and #7 below.
Given what you've told us, your likely culprits (not in any particular order...) as all three are totally valid:
Neighbors: Anywhere in the building, this CAN BE the cause. Talk to others or observe. How you proceed in this area is up to you, but it is possible.
- Structure food internal: Again, anywhere in the building (something may be dead in the wall/ceiling areas), but also #6 (Neighbors) still applies - as they (flies) move around the interior of the structure, they will find any exits they can as they look to explore and expand. Again, do any neighbors (admit) to seeing them too? It can help locate them.
- Environmental exterior: Hallways (via your DOORS) and windows. Are you leaving your windows open in summer? Screens are not perfect. But of all three options I rule this one the least likely, especially since you mention winter activity.
Talking to your neighbors and your building management can both be intimidating or offputting but it doesn't mean it wont help either. It's up to you. Also, it is possible you can walk your halls and multiple floors to see if you yourself can spot a marked increase in their presence anywhere. Not sure you want to do that either, but if it bothers you enough, consider some level of investigation.
Visually inspecting all of the possible entry points, and taking steps to seal off any cracks/openings is good for energy efficiency and stopping flies. Hopefully some of these examples will help you. Aside from that I don't see how anyone here can tell you where they are coming from.