I made this image comparing Necron design elements (mostly GW minis, 1 piece of homemade terrain, and also some DOW1 screens) with the design elements of the new tileset/map we saw in the latest Ork gameplay footage from IGN.
You can see things match up quite closely. Now keep in mind this Ork mission takes place in the "Outer Vaults" (sounds like a place where Necrons would like to hang out). What lurks in the "Inner Vaults" which the Orks are heading to at the end of the mission?
If Necrons have not yet been revealed to the player at this point in the campaign, it would make sense that there are no overt symbols like Necron head statues etc, just more subtle design cues.
There's also a lot of other evidence for Necrons in the game.
- Both previous DoW games have had a secret "big bad" enemy towards the end of the campaign in addition to the standard SM, Orks and Eldar. It was Chaos in DOW1 and Tyranids in DOW2. Orks don't really have the "big bad evil" vibe, and Dark Eldar are more a nuisance than a major threat. That leaves Necrons.
- Before DOW2 came out, Relic kept Tyranids secret until after SM, Orks and Eldar had been revealed. They even cut them out of the cinematic trailer, only showing the full trailer later.
- The story of DOW3 reads "supremacy must be suspended for survival" meaning races must temporarily put aside hostilities in order to survive some greater threat.
- The story of DOW3 has similarities to the story of Winter Assault: The races all go to an icy planet searching for a powerful weapon (a Titan vs the Spear of Khaine), only to find a greater threat (in Winter Assault it's non-playable Necrons).
- Just before DOW3 was announced, Relic added the Necron Lord to DOW2 Last Stand, a game which had not received any new content in years.
However on the other hand, Relic has been firm about there being only 3 playable races at launch. If they had a 4th race, why wouldn't they want to advertise that as a selling point? So if Necrons are present, perhaps they are campaign enemies only and not playable at launch (ala Winter Assault).