While you were on your way to your grandmother's house, the Wolf-Taur you had sex with arrived at the house first, and was still carrying the pair of panties you left behind during sex. While he arrived, the Wolf-Taur opened the door, and saw your grandmother's clothes on the bed, and heard running water from the bathroom.
Your grandmother must be in the bathroom, while the Wolf-Taur was wondering how he would give the panties you were wearing back to you, while you were on your way to her house. Just then, the Wolf-Taur thought it would be crazy, but it had to work as he took all of your grandmothers clothes, and placed them on over himself. Once the Wolf-Taur was dressed, he got under the covers, while hiding all signs of his original identity, and keeping the panties hidden under the blanket.