Black mold on window frames and rust on fittings

Hello everyone,

During the inspection of a property built in 2004, I noticed a relatively pronounced (suspected) mold growth inside on the opened window frames (difficult to describe, see picture) as well as significant corrosion on some fittings.

I am not familiar with such occurrences in our apartment, which is 20 years older. Should this make me suspicious? What could it indicate?

Many thanks!

I could imagine that moist air from inside constantly escapes through the (closed) window and the moisture condenses in the frame. Maybe the window is not sealed or never properly closed.

I also see no seal on the frame. My windows have seals in the frame and the sash. Not that these were removed here "for permanent ventilation."

Are there window rebate ventilations?

No, there are no window rebate vents. It was in a child's room, so certainly not a planned permanent ventilation. It concerns windows on both the front and back sides of the house. So, for example, not only on the weather-exposed side.

Before I start my own topic, I’d better write here, the title somehow sounds appropriate: with the current sub-zero outside temperatures, we have condensation water IN the window frame every morning in the upper floor (not at all on the ground floor!) in the new building (KfW40.....), that is, after opening the sashes, and all around/top bottom left right (photo). But never on the inner panes – the controlled residential ventilation apparently works well from inside. But why in the window? Does this indicate insufficient sealing to the outside? Unfortunately, there is already some mold beginning to appear. We also additionally ventilate the sleeping and wet rooms by shock ventilation. (It should not be due to the fly screens – the condensation water is on all windows in the upper floor, also on those without fly screens.) Best regards