It seems likely that the higher ranking the necron, the more personality they retain. I'm a bit suspicious of this since loyalty to the C'Tan was pretty hardwired before they became necrons, but with only 4 C'tan left it seems more than likely that there are some forgotten enclaves, and that these may have begun behaving oddly (by their standards).
I'm not sure about the partly necron operative idea, but someone with some implanted necron tech, possibly someone who had been experimenting with it himself, seems possible. Very few people would sanction anything of this sort, but the mechanicus does have a history of trying to get its hands on 'archeotech' on necron sites.