In the situation where someone is at the front door and wants to see B,
A: There’s someone here to see you.
B: Oh, thanks. I wonder who it is.
In this situation, what should you say ‘There’s someone here to see you.’ in Japanese?
Casual
B：あなたにお客さんよ
Note: This is not a word-for-word translation.
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A word-for-word translation
B：あなたに会いたがっている人がいるよ
It’s grammatically correct, but we don’t usually say it in this situation.
An exception to this is when you want to tease B in the case, for example, if someone who B likes is there.
We usually say あなたにお客さんよ.
It’s a bit more polite way because the person is in front of you and can hear what you’re saying.
If you hear someone knocking at the door and you don’t think it’s your guest, you can say 誰か来たよ to the other person in the house.
The literal meaning of this is ‘someone has come to the door’.
Kotoba no Hibiki 