A sleeper could have also been recruited, but chances of that happening are very low and the Mafia would be extremely lucky. Mafia Poisoner is a possibility. The Mafia could have used the role to confuse the town and put everyone in further darkness for another day. It's a great role for deception and confusion.
Remember: Next Night we could have an "extra" death. Remember that one of the kills next Night might be due to a poisoner.
Also I'm not sure about the Mentor hard-on here. You people take it way for granted there's "likely one". That would mean the mafia + 3rd party has a potential of becoming 6 people. That's very high for such a medium game isn't it? I think it's extremely likely the Mentor (if any) in this game would start alone, which would explain no kill from the 3rd party because of using the night to recruit, but still. A serial killer is still a possibility.
For example if: Mafia has a Poisoner and Serial Killer hit a doctor-protected target. Evil is also known to use Lovers. We could have Lovers in this game, and thus only one killing party (mafia).