Can flies enter the house through controlled residential ventilation?

Sometimes the supply air filter in the controlled residential ventilation looks pretty wild ;-) But from the valves in the room or their filters, nothing animal-like can be found. We also occasionally have an issue with these little flies. However, that is more due to potting soil or fruit! There are stickers for windows or "signs" to stick into the flower pots. They have pheromones and a sticky layer, cost a few euros, and they work.

But that is more likely due to potting soil or fruit!

I dare to rule that out.

We don’t have these flies anywhere near the plant (there is only one on the ground floor) or the fruit. They are in the bathroom. And they keep appearing there.

Therefore, I assume that they may indeed have come in from outside, but settled somewhere in the pipe...

But there is exhaust air in the bathroom. I don't believe that such small flies can work their way against the current there.

You only have one plant in the house? Wow!

It is much more likely that the flies come from the potting soil than from the exhaust vent of the [Kontrollierte-Wohnraumlüftung] in the bathroom! No fly stands a chance against that.

Set up a vinegar trap. So vinegar with a drop of dish soap in a bowl. If the flies go in there, they are fruit flies. If not, something else. With fruit flies, you can rule out that they come in through the system.

I find it very unlikely that flies enter specifically through the exhaust air in the bathroom. They are more likely to be in the room and are drawn there.