Visible crew/equipment: In the library scene, when the ghostbusters are walking towards the old lady to ask her questions, if you look behind Ray's head to the right you can see a set light with a white tarp in front of it on the second floor of the library. (00:07:35)
Visible crew/equipment: When Dana arrives at the Ghostbusters' office, a crew member can be seen moving behind the filing cabinets behind where Janine is sitting. It could be Venkman, but even after Venkman walks over to Dana, there's still a rogue hand there, showing us it wasn't Venkman and was a crew member. (00:22:10)
Visible crew/equipment: Bill Murray and Dan Aykroyd's stunt doubles are visible in the shot (from behind) that shows the Ghostbusters falling into a hole. (01:20:35)
Answer: He places it on the altar just after they believe they have vaporized Gozer with the proton packs. At this point, thinking the ghost to have been destroyed, he sets it down as he believes the job is done and he will not need it.
Jazetopher