Sean Ambrose: You know women, mate. Like monkeys, they are - won't let go of one branch until they've got hold of the next.
Ethan Hunt: You turned around.
Nyah Nordoff-Hall: What are you going to do? Spank me?
Sean Ambrose: In just a few hours you can be assured of going down in history as the typhoid Mary of "Aus." G'day.
Answer: Ethan was the most probable candidate. Remember that Ambrose and Ethan had specifically worked together a few times with IMF in the past, as stated in the film, with them body doubling each other. Once Ambrose had his suspicious raised by the envelope, he decided to test Nyah. So showing up to her with Ethan's face would indicate if she recognized Ethan, or thought she was being approached by a total stranger at night.
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