I mean, it's possible there's an SK and no real mafia faction, but that seems incredibly unlikely.
Where have you been? I think everyone here has suspected a SK since Night 1.
Doubt only comes in when you suspect faust was lying about their faction not using their kill on Night 1. I'm inclined to believe that they didn't, seeing as they had(have?) a single kill for the whole game, and multiple players that can ensure it will be used. Plenty of time to figure out who to use it on.
Sounds like I've been misremembering, which isn't too unlikely given that I lost engagement a little at the start of D2 just through being busy, and now I'm rather unwell and kind of fuzzy-brained. That makes me rather more autistic than normal, in a detail-oriented sense, but also rather less able to remember good levels of detail. My work day pretty much sucks!
Anyway, I thought we had one claimed vig one-shot (you), one kill from the Alien faction, one from the WW, and then a presumed fourth from Mafia.
If your own numbers, even if you're ensuring at least one vig being present, are also now indicating a high likelihood of there being an SK, then either one of the major expected scum factions didn't shoot after all, or something saved us from a fifth expected kill, if there was an SK shot in the mix.
I'm perfect able to accept any of those things as an explanation, but I do still find it strange that you, who have presumably had to think through this in detail in order to code up your sampler, hadn't felt driven to mention any of the oddities or weird scenarios that that reading of the data implied.