Dark discolorations on door frame - Where could they come from?

Hello,

recently, the stains/discolorations shown in the photos have appeared. None of the residents of the apartment can make sense of how these could have occurred. It is also completely unknown when/over what period they appeared.
My question is, could this possibly be a known issue, or can someone provide a valuable tip regarding the cause based on the pictures?
Brief description: Dark discolorations on the lower edge of the frame, does not seem to be paint, cannot be affected by wiping or scratching. Seems to be somehow "in" the wood (decor).
Attached are two photos of the said spot, one with flash and one without.

Best regards
N

We have similar stains in our rental apartment that allegedly appeared during the installation of the frames due to glue / foam. They can no longer be removed.

Were the doors newly installed?

Were the doors reinstalled?

They don't look new. Can it be wiped off?

No resident of the apartment can make sense of how these could have come about.

Says who? Roommates also deny it.

Maybe someone used a too aggressive cleaning agent...?

No, the doors are not new. All about 10 years old. You can’t just wipe it away. (otherwise there wouldn’t be a problem ) Interesting ideas, especially with the aggressive cleaning agent. Could moisture rising from below also be a possibility, causing the stain to develop very slowly over a long period?

I can't imagine that. Moisture tends to spread more evenly, so there should be a semicircle or something similar visible there. In the pictures, it looks more like a longer streak.