
Continuity mistake: In the car chase scene with Nyah after they split and hit again and are spinning look at Ethan's car. The driver's side in the close ups was on the American side, then in the spin it is reversed to an English side. (It is a negative image - the license plate is reversed). The next image is back to normal.
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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