Or HaChaim on Genesis 18:9/אור החיים על בראשית י״ח:ט׳

1 ויאמרו אליו איה שדה אשתך, They said: "Where is your wife Sarah?" The angels wanted that Abraham should realise that the message they had to deliver was primarily for Sarah; this is why they enquired about her whereabouts. Abraham indicated that she was within the tent ready to listen to what they had to say to her.