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If the necrons ruled the galaxy before humans, does that mean that any planet near terra could be a tomb world, or has the imperium made sure there are no tmb worlds nearby

I have a feeling that they would have been discovered long ago.
The majority of necron tomb worlds are in one sector of space, largely on the outer reaches of the galaxy. Yeah, Segmentum Ultima

They probably used different planets as home base. Like they could have centered their empire in what is now the fringe, but back then it was the center of everything.

While that may be true, if I were an Imperial citizen who was "in the know" about the existence of Necrons, I'd definitely turn a very watchful eye on any arid, desert world I came across.