Can flies enter the house through controlled residential ventilation?

I don't know much about it, but to me it looked like a "real fly"/housefly at first.

The picture is highly magnified; with the naked eye, you can't even see the wings—they are really very, very small.

The little creatures only appear in the bathroom. There are no plants or fruit there. They weren't present for the first approximately six months after moving in.. But for now, I'll first clean the drains with vinegar and then see what happens in the future..

Do you possibly have a bathtub or another drain that you do not use? We had the same for a short time, until I noticed that the siphon of the bathtub, which we never use, had dried out. Now the bathtub regularly gets a little bit of water and everything is fine.

Do you possibly have a bathtub or other drain that you do not use?
We had the same for a short time, until I noticed that the siphon of the bathtub, which we never use, had dried out.

The drains are always in use, our daughter uses the bathtub for showering.

That is why I first came up with the idea of controlled residential ventilation, since nothing else in the bathroom indicates it..

Let's see if it can be fixed by the vinegar rinse.