Yes, bedrooms have the windows open more often, and usually less/no heating. How many hours a day do you have the windows open? Is there a heating source on or nearby? If you close the window, you'll see RH drop quite fast (in most houses). I'm going to try this same experiment tomorrow, I'm quite interested to know.

Still - mold in houses is not a problem in houses in Western/Northern Europe, except for the most pathological cases (for example, mold scores you iirc 2 points in Belgium on the 'uninhabitable property' test, where 9 gets your building declared unfit for living in. Having small holes in the roof or small cracks in the walls (still structural defects) gets you 3. That's how unusual mold is around here. OTOH when I lived in New Zealand, mold was met with 'eh, there's bleach in isle 5 at Countdown, just wipe it down.'. I was like 'wut?'.

> If you close the window, you'll see RH drop quite fast (in most houses).

Alas, it does not, but that's possibly because other windows are open around the place and there's relatively good airflow.

> I'm going to try this same experiment tomorrow, I'm quite interested to know.

I'll see how it goes tomorrow with the window closed and various doors closed to minimise the effect of the other windows.