And those who apparently hadn't known were Ross, Jasmine and Evie. All three were banished so everyone then knew that Ross was a traitor and Jasmine and Evie were faithfuls.
There was a murder (Zach) after Ross had been banished. Therefore there was definitely still a traitor (weirdly no one seemed to realise this last night!)
As there was another traitor apart from Ross, it blew apart the shield theory because a traitor would have known Harry had the shield and therefore they hadn't tried to murder him. Therefore there must have been a recruitment instead.
Whilst it doesn't make Harry a traitor necessarily, it does look more suspicious. A bit of a coincidence...