Place it at the cut where the unexpected person first appears, then follow it with a Vine Boom to underline the reveal. For a slower build, put a Dramatic Dun Dun Dun just before it so the question answers the tension.
關於此聲音
This is a bare, just-over-a-second voice clip of someone demanding to know who the person in front of them is, delivered in a blunt, suspicious tone with no music behind it. It feels startled and funny rather than hostile, like catching a stranger in your space and blurting out the question before thinking. It circulates as a short-form reaction clip for surprise entrances.