Derek could see the image so plainly--Isaac's slender fingers stretching Stiles' hole. He let out another involuntary groan and swore under his breath, dragged his fingers through his hair. He could stop by Isaac's room and let him know not to come by, but...no. This could potentially be good for both of them.
He shot Stiles a glare over his shoulder. "See that you are," he threatened. "And don't come. I'll know if you do." He tapped his nose in warning.