Spy dust is a substance that can be used to track the movements of anyone who touches it. It was developed by the Soviet Union in the 1960s, and they applied it to door knobs, car floor mats and other surfaces to track the movements of Americans living in Moscow. Once someone touched the substance, they would smear it on every subsequent surface they touched.
The US government got a sample of the Soviet powder (containing both luminol and a substance called nitrophenyl pentadien) in 1984. The revelation that this ?spy dust? existed caused a scandal, not because of the tracking, but because people were concerned about the potential health hazards. Scientists studied the substance and determined it safe for external use at low levels, and useful as a "useful shadowing pursuit," so let this be a lesson to wash your hands after you touch any door knob.