I had been struggling with this mystery for quite a bit... until I noticed, it was actually very easy to solve. But then I placed it on a pending list, for a good occasion to come here.
Today I could combine a few mystery for a nice early winter walk. At ground zero, it took me a bit of looking, as my GPS was not the best. And it turned out, that the box is actually frozen on it's spot at the moment. I was considering sending a picture to the owner, when I realised, I could still open it easily and retrieve the logbook with no issues. But it is good I didn't come here in heavy snow, that would have been trickier.