Derek was checking over the animal pens, Batman's leash looped around his wrist since the puppy was mostly just sniffing the ground, probably trying to find something to eat that he shouldn't. He snapped to attention at the commotion out front that immediately sent Derek into panic mode as well, until he recognized the all-too-familiar voice and suppressed a groan. It quickly turned into a yelp when Batman took off - for a puppy he was already strong, and he tore out of Derek's loose grip easily.
"Goddammit," he mumbled, jogging after the pup, who was jumping all over Stiles. "Be careful, he just got out of the hospital, don't be too rough with him."