A smoking gun is shorthand for "an artifact that will prove beyond a reasonable doubt that the Niku hypothesis is correct."
The image is taken from a scenario in which a suspect is apprehended shortly after a shot has been fired with the "smoking gun" still in hand. One would hope that such evidence would prove to any jury that the suspect found holding the smoking gun was responsible for the shot that was fired.
In 1992, TIGHAR proudly announced that "We Did It." Subsequent analysis by TIGHAR showed that it had not yet found the smoking gun. The navigator's book case turned out to be from a WWII aircraft and the piece of alclad aluminum has not been definitely traced to the Electra.