Chapter Four: The Sauna Test - S3-E4
Visible crew/equipment: When Mike and his friends are discussing the Mind Flayer needing a new host, El asks how they'd be able to tell if someone is a host. Then it cuts to Hopper and Joyce pulling up in front of Town Hall, and before Hopper gets out of the car the boom mic is reflected on the squad car's window. (00:14:25)
Chapter Five: The Flayed - S3-E5
Visible crew/equipment: While Bruce chases after Nancy and Jonathan they hide in the OR, then Nancy picks up the red phone and shouts, and in the following closeup of the ceiling mounted domed light, the upside-down reflection of a crew member wearing shorts and sneakers can be seen quickly. A few minutes later, after Jonathan jabs scissors into Tom's throat, when Jonathan leans back out-of-breath there's another closeup of the surgical light, and we can see the upside-down reflection of the cameraman's arm and part of the camera as it moves closer, then it's visible again a few shots later. (00:43:10 - 00:46:40)
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.