I walked around the whole building today.
There are a few doors—most of which do not really look like doors—but no housenumber printed anywhere.

Correct me if I am wrong but it seems that current practice is as such:
– If a number is visible on the ground, we add it to OSM
– If a number is visible in UrbIS, we add it to OSM (with a not:* prefix if actually does not exist)
– If a number only exists on paper, i.e. people or business claim this is their address, we do not add it.

I am afraid this is the case here.  What do you think?