Necrons are an enigmatic and really appealing race, both in how the miniatures look (frightening killing machines without remorse nor pity for the other races) but also due to their background, so I try to read as much as possible about everything related to them.
One curious point that only adds more mystery to them is when Games Workshop really created the Necrons.
Sources say that in 1998 the first miniatures appeared (Necron Raiders), then getting a full army treatment after releasing the third edition of the main game.
Old Necron Raiders, from Mis Olvidadas Figuras
Actual necron miniatures
But, being an old dog like I am, introduced to "RPGs" at an early age because of HeroQuest, I was trying to remember why those Necrons were familiar to me from earier...
And then I remembered: Space Crusade (aka Star Quest in some countries) was my second boardgame, and was also done by Games Workshop to introduce the Warhammer 40k world to new players (and then try to move them to proper fields with the normal miniatures and games). Exactly like Heroquest but for the futuristic setting.
I won't enter into details because you can just navigate to the wikipedia link and read by yourself, but I will point out one date: 1990. The date where it was released.
And now, look at how one of the most dreaded monsters the game had looks, the so called "Androids":
Space Crusade painted Android miniatures,from the great Zen StarQuest
Don't you agree with me that the similarities are more than noticeable? Excepting the glowing energy bar in the weapon... looks like almost the same model!
We are talking about 8 years before the official first appearance (in a White Dwarf).
It looks quite reasonable to think that GW first thought of a futuristic, alien version of the undead skeletons back in 1990 (probably early while designing the game), and when they wanted to create a new race around 1997 they just went back and redefined the "Androids" (and gave them a better name).