Sounds like the old crime committed with a bullet made from ice water trick.
The modern version of the ice bullet would be one that disappears when it contacts blood.
I seem to recall an episode of the “Untouchables” were the mobsters were doing their books and accounting on flash paper. When the Feds burst into the back room, the mobster took a cigar and touched the “books”. Flash, gone in a second.
Once saw a movie where a murder weapon was a hairbrush handle made of a material which, when put into boiling water became pliable enough to shape into a sharp point and when put back into the hot water, automatically resumed it’s original form.