ᐅ Stain development on wall – No moisture detected

Hello everyone,

We have been living in our newly built home for a year now. On the ground floor, specifically in the hallway area, two spots have slowly appeared along the edge where the side wall meets the ceiling. About 2 cm (1 inch) by 25 cm (10 inches)... slightly beige/brownish. My moisture meter, as well as one used by the tradesperson who installed the pipes, bathrooms, and heating back then, showed no moisture at numerous tested points in the walls. All inspections on the upper floor, for example in the bathroom with sink and bathtub, bedroom, etc., show no damage and no moisture on floors or walls anywhere.

Now my question: what could this be?
We have a ventilation system. Plus, it was an extremely warm summer... so it’s not really an environment where mold should form — if that’s even what it is?!

Which device or test strips can be used to further test these spots?

Thank you very much for your ideas and help!
Regards,
franzl2000

Strange. Have you tried scratching it a little? Is it wet? Rust? Dab some oxalic acid or citric acid on it with a cotton swab. If it then disappears, it's more likely rust.

Nine, I haven’t scratched that area. I can get some citric acid and see if it removes the stain.

It’s really not wet there at all.
Where would rust come from if there are no metal pipes inside the walls?
Regards

We will investigate that once we know what it is. One step at a time.

Who knows what was sealed there during construction. Some kind of bitumen-based sealant or similar, probably because nothing else was available at the time. Now it is slowly diffusing outward.

Doesn't look good... what’s the current status? Did the citric acid help???