Waste cores from those bore holes have been found scattered all over Giza and other megalithic Egyptian sites. The tool marks on them match the tool marks inside the bore holes.
I don't think anyone paid much attention to them until the advent of modern core drilling technology. Only then did some people look and notice the obvious.
Who knows if we'll ever figure out this mystery, but there's a smoking gun in the rock, that's for sure.
Ethiopian in the celluloid fuel supply...