I never got round to taking a cloe up look at this monument(?) so I don't know what it is, but it is on the north bank of the Nieuwe Maas river, Rotterdam.

it seems to be mostly invisible.

sef_lopod: yes! The sunlight was so bright that it seems to have saturated the sensor so that the sky and the monument are the same colour. I have another photo where the same sort of thing causes an even weirder looking effect.