Just an update.
I contacted NEC support here in Spain and, as expected, they say its a "panel problem" "not repairable due to high cost" but they gave me their German workshop contact in case I want a "repair estimate"... Well it seems it will need a "little" harder pressure
In the meantime I made some investigation in the web and find some interesting things seemingly similar/related to my problem, enough to enlighten a bit what type of issue happened to my monitors.
Basically, it´s a non-uniformity problem that seems to plague LCD technology called "Mura" effect. I find two pages witch I think are closely related with my exact problem:http://www.avsforum.com/t/1467644/lcd-dark-blotches-mura-clouding-persistence-what-to-dohttp://www.behardware.com/articles/589-1/panels-a-carte-mura-components-dead-pixels.html
What it's not so clear to me at the moment is the possible solutions. In my case the problem seems to be a Type 2 dark blotches mura (according the classification on the beharware page) and I suspect it maybe it´s related to problems due to dirt (least probable) or heat deformation in back diffuser or polarizing layers, or to some type of ionization of the LCD. Why I think so?... well it´s only a guess but my principal clues are their wavy shape, the fact that they are more noticeable some time after switching on the displays, and the evolving nature of the dark areas.
I've taken new photos of the problem (one and a half months between them) and it as clearly changed moving towards the center of the screen, diffusing and leaving relatively clear areas it was affecting before.
Anyway any help will be welcome about confirm/rebate my guess or what to do to improve/fix the problem. Thanks in advance again