Swabs taken from Conservative AM William Graham's hands at the Welsh assembly building revealed traces of the drug, probably from a door handle.
He had arranged for police to come in to demonstrate the hi-tech machine.
Social Justice Minister Edwina Hart's hands were similarly cross-contaminated also bringing a positive test.
Mr Graham, a long time campaigner on substance abuse issues, said: "I can't think where I could have got it from".
He had arranged with Gwent Police, who had recently bought the first Ion Track drug detection system in Wales at a cost of £40,000, to demonstrate it to assembly members in Cardiff Bay.