In this case, probably the manifestation of murder. Considering the story that Cain gave Saturday and the fact that she apparently knew the number of people that Washington and Ten Ten had killed.

[He knows very well that not all killings are murder. But that doesn't mean that a spirit like this would make a distinction.]

If she's summoned, it could also explain why the deaths only happen in short bursts as well. Just enough to get people riled up but not enough for the company to really take notice. After all, this is basically a moving city. Deaths happen in cities all the time. And if they can identify other reasons for the deaths, reasons that don't include them being murdered? So much the better for the Rig higher ups.