Janine Melnitz: Do you want some coffee, Mr. Tully?
Lewis Tully: Do I?
Egon Spengler: Yes, have some.
Lewis Tully: Yes, have some!
Winston: I gotta tell you, these things are real. Since I've joined these men, I have seen shit that'll turn you white!
Peter Venkman: I get it! Oh! Very cute. Whatever we think of. If we think of J. Edgar Hoover, J. Edgar Hoover's gonna appear and destroy us.
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