Chapter One: The Hellfire Club - S4-E1
Continuity mistake: Tammy Thompson's voice changes octaves as she sings the National Anthem. At the start, she's in a normal mezzo range. When the camera cuts from her to the back-and-forth dialogue between Steve, Robin, and Vickie, her voice then changes downscale to a very noticeable alto.
Answer: The body was fake, made by the scientists at Hawkins Lab in an attempt to cover up his disappearance and the existence of the Upside Down.