Who overheard Isaac sending Esau to hunt game before blessing him?

Answer: Rebekah

Rebekah was listening when Isaac spoke to Esau, and she told Jacob what she had overheard.

“Now Rebekah was listening to what Isaac told his son Esau. So when Esau went into the field to hunt game and bring it back, Rebekah said to her son Jacob, “Behold, I overheard your father saying to your brother Esau,”— Genesis 27:5-6 (BSB)