I'll go ahead and say this now. I'm a spy. Been following Onion every night, except the first night when Link asked me to follow mathman (I didn't trust Link so I followed randomperson instead, which is why I voted on randomperson that first day phase). So far every arrest Onion has claimed to make has been true.
So let's say Onion is mafia. Every time he arrests someone he reveals they aren't mafia. If he lies and says he arrested someone and a night kill happens, they are cleared of mafia status anyway. If he continues to not arrest people, it becomes increasingly obvious that he's mafia. So if Onion is mafia, the truth will come out eventually. My job as a spy is pretty much done, as the spy can't see night kill votes.
If I were mafia, I would have killed Onion last night phase, knowing that he can clear people of being mafia. I don't think there was any way to know who was the nurse, killing the nurse was likely just luck on the part of the mafia (and submachine drawing a lot of attention to himself). Knowing that, I think the level of suspicion on me should be extremely low.