Necron Sweep attacks specific say they ignore cover (which may be what you are thinking of). In the rulebook it only says close combat attacks and template weapons ignore cover saves as well as anything that specifically says they ignore cover.
So it doesn't officially say you get to take cover saves, but unless your opponent is arguing that they are a CC attack (in which case Wyches get a 4++ to it), then there isn't any real evidence to support the arguement that you wouldn't get it.
Also keep in mind it doesn't specifically say when you do get to use cover saves, including from shooting so you have to assume that until GW says otherwise you get then in situation that doesn't say otherwise
The worst sort of protection is confidence. The best defense is suspicion.