I'm not sure if I understand you, but I think this may be at the root of the difference in interpretation. I'm assuming that by "it" you mean a threat, and because you've used a neuter pronoun I assume that you aren't talking about the person you've been engaged in conversation with. So just what constitutes a threat? To me, a threat is always a creature with which you are about to be engaged in combat. In the case of a negotiation that turns into combat, the threat is the NPC that is trying to stab you with a dagger. She has obviously already been noticed because you were having a conversation with her just a moment before. But from your question here, you seem to be saying that the dagger is a threat, or that the NPC's next move is a threat. I don't know how the rules cope with such an interpretation, but it differs from mine. My understanding is that the NPC herself is the threat, and she has been noticed. It doesn't matter whether the PCs consider her to be a threat before she attacks. She quite literally is a threat, and the PCs are quite literally aware of her.