Alright, this is right out of the Necron FAQ.
"Q. If a unit of Necrons falls back after taking casualties, how best to keep track of whether they are effected by a resurrection orb or not?
A. A Necron unit falls back, having been beaten in close combat by a unit with power weapons. However, there is a resurrection orb nearby. To keep things simple, when the casualties are suffered, assess whether they are in range of the resurrection orb. If they are, lay them down and move them with the unit. If they are are not in range of the resurrection orb, remove them entirely."
That seems pretty cut and dry to me. Necrons who have been wounded and are waiting thier WBB rolls move with the unit they were in when they suffered the wounds. Downed Necrons still belond to the unit they were wounded in. You have to take the good with the bad. If that same unit described in the example given in the FAQ was cut down by a Sweeping Advance, the wounded Necrons would also be cut down since the FAQ clearly counts them as being part of the unit.