‘And Rebekah heard when Isaac spoke to Esau his son. And Esau went into the open country to hunt for venison and to bring it.'
There was no reason why Rebekah should not have been in the tent when Isaac spoke to Esau. The giving of a blessing was not something that had to be done in secret. On the other hand she may have been lingering around outside, knowing what was on hand. As she saw Esau depart to carry out his father's wishes her mind was racing. She no doubt remembered the promise made at their birth that the elder would serve the younger, and she wanted the blessing for her favourite son.
When we consider her next actions we should also consider that it seems that Isaac has no special blessing for his younger son. Both sons deserved to be blessed, but Isaac apparently thought only of Esau, and he certainly ignored what had been said at their birth. Furthermore the sale of the birthright was a legal fact and it is unlikely that Isaac did not know of it. But he considers he can override it (as his blessing demonstrates). How unfair people can get in old age when they are unable to help themselves and must look to others for everything. Rebekah on the other hand feels she cannot allow this to happen.