Tues Sotah 36

When Yosef returned to the house, he saw his father's image in the window.

Toafos quotes R' Moshe the Darshen: this Drasha was learned from the following implication. The Torah writes that there was no man from the household there. This implies that there was someone not from the household, which would be Yaakov.

However, Tosfos asks: where do we see that he was in the window?

Tosfos answers: perhaps because the Torah says there was no one etc. in the house, implies there was someone just out of the house.