As I mentioned, I have opened can of worms and have found some unexpected contamination.
First by careful examination using a Mini Quasar led lit magnifying glass I noticed several white coloured specks, one of them being much bigger than the rest. I first tried the blower with no results; the specks or botches were firmly fixed to the sensor protection film.
I went ahead and using FX sized swabs and the VDust Plus fluid I managed to "dissolve" them (instead of removing), that's why I immediately thought that they were not dust specks but more like grease particle impacts. I then noticed some overgreased mirror mechanism that could well be the origin of the contamination. Also noticed that the big botch on the sensor was white, the same colour the grease has, so eventually I will inform Nikon Europe about my findings and ask them what the best option is in this case. (I know we are not supposed to "touch" the sensor, but I need the camera now that I sold my D3s, so I will just mention the dirt...) The specks are gone now, but the sensor has very slight film like patches in some small areas where the swab was not able to decontaminate it completely, and apart from anything I did not want to insist. I include a couple of pics for you to see and comment; might have to enlarge them if you want to see anything.
One thing is clear; if it is grease, then it is a manufacturers problem and it leaves me wondering how long this grease spread will continue... I have read some reports of this same problem on other D4's and D800's, but that does not help me much (somewhere in "NIKON RUMOURS" page).