Mold behind the radiator on the exterior wall

I wouldn't suspect the basement either. A damp basement floor is normal and not a problem. It's the same for us. It has been working without issues for 102 years. If it's a niche and hasn't been heated, then - if it is mold at all - it's probably not a big deal. If you want to be on the safe side, you could send in a sample. What are the further plans? If it is going to be renovated anyway, the niches will probably be bricked up and new radiators installed or a surface heating system put in?

It's reassuring that it's similar with your basement.

There is still some debate about whether we will close the niches or leave them. There are 2 radiator niches in the room and of course, that takes the radiators out of the way and creates space. If anything, there will be new radiators; we do not want to install a surface heating system.