The Necrontyr may have had some sort of "soul" before they became the Necrontyr, but I would hesitate to describe it as anything more than a nucleus of the warp souls that the psychic races the Old Ones created.
A warp soul being a psychic pattern that seems to "align" warp energy with it, akin to a magnet and iron filings perhaps ? its a limited analogy, but I think it gets the idea across that the more warp souls that exist that have a specific range of ideas and emotions, and the more powerful individual souls are, the stronger warp entities that are attuned to that range are. When a soul leaves its physical "host" it joins the larger pattern that it is aligned with.
The psychic potential of the necrontyr was clearly extremely limited, as they weren't y'know, exactly legendary psykers
Plus there is the unescapable and supported fact. C'tan feasting on a sentient organism do not consume the warp soul, they EXPLICITLY do not consume the soul, upon destruction of the host body, and absorption of its physical energies, its "essence" to quote the Deciever, which appears to be some sort of bio-chemical potential energy, perhaps combined with natural electrical fields, etc .
The Souls of the Necrontyr, as Ancaris has proved via evidence, (note theres another little example, when Abaddon interrogates a Daemon as to the contents of the "temple" in the IIRC Noctis labyrinth on mars) MUST have entered the warp and been consumed by the entities there, or simply dissapated due to their probably extreme lack of psychic energy...lets face it, in Farseer by Bill King, Auric, a Farseer describes the average greater potential of the Eldar soul allowing them to survive death as a sentient soul, and suffer unending torment due to their lack of any "pattern" or diety attuned to their pattern apart from Slaanesh, unlike the average human.
If the average human has more psychic potential than the average Necrontyr...VERY VERY likely indeed, then the Necrons souls will simply have dissapated.
whats left of the Necrontyr is some sort of energy pattern, that has been loaded into whatever technology serves as hardware for the Necron constructs. Its actual a rather apt method of cruel abuse, pretty appropriate for the C'tan, rather than simply programming the constructs, you transfer an operating system and some rudimentary imperatives, i.e. obey, harvest, etc etc, to the hardware, an electrical simulacrum of the Necrontyr, getting rid of useless memories and desires, as well as something probably quite elusive, and potentially dangerous to the necrons in their closest servants. Vastly reducing the potential for subversion via psychic means, (obviously machines can be psychically manipulated, but if you are powerful enough to telekinetically fuck with something like a Necron construct, its pretty likely you could bugger up an organic with barely an effort)
in summary.
Proof has been provided of the Necrontyr having limited in comparison to human psychic potential,
Proof has been provided of the Necrontyr being consumed by the C'tan, in such a fashion as to destroy their physical forms, and cast their souls adrift in the warp, while retaining a wholly reduced version of their intelligence to use as a method of animating technological creations.
Given the limited potential of Necrontyr souls, and the comparative example of humans NOT remaining sentient after death unless of above average psychic/mental strength, the likelyhood of said souls rebinding to the Necrons is low at best, and would require proof to counter the weight of evidence against it.
However, the problem remains of the Pariahs, also described as "souless" much in the same fashion as Necron constructs hosting the mentality of former Necrontyr. (i.e. everything classed as "necron")
Necrons do not have an untouchable quality, which would immediately point to two conclusions, That the Necrons DO have souls and are thus souls and mental patterns in robots, or that they are low-level untouchables...
Just to note, I think Drachenfels analogy of anti-matter and matter as pertains to pariahs and the warp is complete bollocks. does a pariah annhilate when exposed to the warp ?
ummmmmm.....no!'
Pariahs actually seem to function as an insulating or non-conductive material and electricity, warp energy is directed at a "bubble" of the pariah energy, and is unable to pass through, its worth noting that, as with real non-conducting materials, you sling enough energy at em, and they can be destroyed. The Eisenhorn trilogy provides an example of this, with an Untouchable being mentally destroyed by a massively powerful psychic entity, i.e. a Daemonically tainted Titan AI.
The principle of an insulating energy field seems to hold true, another analogy being a Tokamak, a shaped magnetic field retaining plasma.
Example, a psyker slings a ball of fire at you, if its warp energy, its a burst of energy that cannot pass through the insulating layer...if its a "normal" ball of fire, i.e. a light and heat emitting ball of gas, then it is being in some form contained by the application of warp energy to prevent it simply dissapating, possibly the input of energy as well, etc etc, so the ball of fire will simply emit all its energy, and without the artificial aid of warp manipulation, thats all she wrote. a Pariah should NOT be able to stop something using warp energy to ignite something, then telekinetically sling the burning object at a Pariah, unless the Warp user is within the insulating envelope of the pariah, in which case the Warp user cannot access the energies of the warp, his mental contact severed, in a similar fashion to slipping a non-conductive material between a battery and an electrical devices metal contact.
Anyway, back to the Necron souls and Pariah conundrum.
My theory, to reconcile the given evidence, i.e. Necrontyr description as "souless" Necrontyr being consumed and transferred by the C'tan, the consumption process universially described as setting the souls adrift to be consumed by Daemons, Necron constructs operating with Necron mental patterns NOT having a Pariah effect.
The Necron constructs with necron mental patterns, (mind rather than soul theory) possess the remnants of a Necrontyr sentience, which in all likelyhood DID make some tiny imprint upon the Warp before being " fed upon " by the C'tan.
This sentience is rudimentary in emotional and self generated ideological attitudes, as well as memory, this entails that it has a rudimentary warp presence, small as to be barely registering. Pariahs/untouchables are described as possessing a negative presence in the warp, i.e. casting a shadow, or dipping an impervious object into an ocean.
as a byproduct of this energy pattern, Necron constructs have a positive presence in the warp, because they DO have ideas, however these ideas are so faint and overridden by an overwhelming priority, to serve the C'tan, and obey their directives and the priorities of their engineered consiousness, that they are probably almost neutral. I would expect low end AIs in 40k to have a similar form of Warp presence, as 40k AIs can, and have become psychic entities. i.e. the Dreadclaw Assault craft, so clearly being organic is not a prerequisite of having a warp presence, however, having a Soul , i.e. a coherent pattern of ideas and emotions capable of collectively and proportionally, individually, exerting influence upon the energy of the warp definately requires more than the Necrons can be proven to possess. i.e. more than a rudimentary operating system.
