Stain development on the wall - No measurable moisture

Hello everyone,

We have been living in our new building for a year. On the ground floor, and only in the hallway area there, two spots have developed very slowly on the edge where the side wall meets the ceiling. About 2 cm by 25 cm... slightly beige/ brownish. My measuring device as well as a device from the craftsman who installed the pipes/ bathrooms/ heating at the time show no moisture in the walls at numerous tested points. All inspections in the upper floor, for example bathroom with sink/ bathtub, bedroom, etc., show no defects and also no moisture anywhere on the floors and walls.

Now my question: what could this be? We have a ventilation system. Additionally, our extremely warm summer... so it is not an environment where mold should form – if it is mold at all?!?!

With which device or test strip can the spots be tested further?

Thank you very much for your ideas and help!!
Regards
Franz

Strange. Have you scratched it a bit? Wet? Rust? Dab some oxalic acid or citric acid on it with a Q-tip. If it goes away, it's more likely rust.

Nine, I haven’t touched that. I can get some citric acid and see if it goes away.

There really isn’t anything wet there.
Where would rust come from if there are no metal pipes in the walls there?
Regards

We will research that when we know what it is. One thing at a time.

Who knows what was sealed there in the construction. Sealant based on bitumen or something, because nothing else was available at the time. It is now slowly diffusing outward.

Doesn't look good... how is the status? Did the citric acid help???