Regarding Necrons:
897.M41 was the first reported contact with the necrons. It´s not beyond imagination that the DW or Ordo Xenos would not report anything on that matter. Especially since they´d probably entirely unaware about the extend of the threat the necron pose.
So we´ve encountered a couple of metal skeleton´ish guardians and a bunch of robot beetles in an acient xeno structure, and now what? Ring the alarm bells at terra? They´d probably wait till they can say a wee bit more than "We´ve found a pyramid shaped xeno structure with some sentinel robots inside on backwater planet whatever-the-heck-4375", before the message goes official.
I wouldn´t bet on their appearance but I don´t think the necs are cancelled out of the setting by virtue of lore inconsistency.