Isaac had settled easily into sharing a room with Derek, but he still found himself rearranging his things, from time to time. He was halfway through rearranging his shirts int he dresser, when he recognized the sound of his Master's footfall, heading toward the bedroom. He quickly shut the drawer and got to his feet, ready to attend to whatever need Derek had. It made him fulfilled and happy, at least until the door opened.
Derek smelled off. Worried, and sad, and...
Isaac froze. He knew Derek's scent as well as he knew his own, and he also knew how that scent changed when combined with his own, or with Stiles'. The scent that assaulted his nostrils was very clearly a combination of Derek and someone else.