The Hebrew says simply that Sarah was listening at the tent door. Targum Pseudo-Jonathan on Genesis 18:10 puts a second listener behind her. And Sarah was hearkening at the door of the tent, and Ishmael stood behind her, and marked what the Angel said.
Ishmael is thirteen. Circumcised in the previous chapter. Blessed by God with his own promise of twelve princes and a great nation (Genesis 17:20). He is standing right behind his stepmother as the angel announces that, this time next year, she will have a son of her own.
The Targum does not explain what Ishmael feels. It simply notes that he heard it. A teenager, freshly marked with the covenant, overhearing the news that a different child — a child not yet conceived — will be the carrier of that covenant. Every tension that will later erupt between the two sons is already quietly seeded in this moment (Genesis 21:9).
And notice how gentle the angel is. The message is meant for Sarah. But the tent walls are thin, the door is open, and everyone within earshot is going to carry this announcement into the rest of their lives.
The Maggid reads this as the Torah's way of telling us that no promise is made in a vacuum. What you announce to one child, another child hears (Genesis 18:10). How you deliver good news to one person becomes part of the story of the person standing just behind them. Abraham's tent is about to be stretched in ways no one has quite prepared for — and the stretching begins with a young boy listening quietly to an angel he was not meant to hear.