Cut to the front of an apartment building (which I'm pretty sure is the same one in which Possibly-Hoofed ManBoy decapitated his uncle [also, it's totally not the same building outside of which Zach Guzman was scouting Ass-Thumper]) where police are rolling out the yellow crime-scene tape. The boys find out from a neighbor that he tried to kill his wife as he is ushered into a police car.
Cut to Sam out back of, presumably, Fancypants's apartment, poking around some more garbage cans. Dean jaunts up to his brother all jaunty, quickly dispatching his own doubts when he tells Sam that Fancypants's story is that he was driving home from a business trip when his wife was attacked and that, furthermore, he saw himself in the house when he got home. Sam narrows his eyes: "Two dark doubles attacking loved ones in the same way." OH LORD. That line is a baby-killer. It kills babies. And possibly kittens. Dean proposes that it's the same thing doing it, causing Sam to respond, "A shape-shifter?" Dean motors through his Monster Hypothesizing -- every-culture-in-the-world blah-diddy-blah -- and Sam answers in Pavlovian kind, listing the types of monsters that fit the bill: "Skinwalkers, werewolves." Jensen Ackles is literally fusing words together in his delivery, so quickly does he want to get this exposition out of the way.
Sam looks up and asks Dean if he knows of any shape-shifters that can fly, because the trail he picked up behind both Zach Guzman's and Fancypants's houses just abruptly ends. Dean says he doesn't know of any flying shape-shifters, and then wins the Nobel Prize of this episode (awarded for the exhibition of sentience) by proposing to Sam that the being could also have gone "down." The boys look down at a steaming manhole. Heh. Hot.
Cut to the brothers descending into a sewer tunnel. Not hot. Sam is excited by the probability that the tunnel runs under both crime scenes. Dean wanders over two feet, and calls Sam over. They kneel, and the camera pans down to show a truly horrific pile of bloody human tissue, with what looks to be a human ear lying in the foreground. Dean uses a knife to scoop up a slimy strand of skin or hair or both, and says he "just had a sick thought. Maybe when the shape-shifter changes shape, it sheds." He thwaps the slimy strand back into the pile and ewwwwww.