As I said I was being too generous to the relatives by looking for a way to absolve them. But I've now come to the conclusion the plan was hatched when DS Jones picked up the silencer from PE and the pair sat drinking whisky during the small hours and/or went over to WHF that evening. This explains why the relatives did not make more of the find at the time and their accounts of who found it, when, its appearance and how it was handled thereafter vary so much.
Many a plan has been formed, and seems workable, where a jar of three has been consumed. However, in the cold light of day, when the head clears, such plans frequently look ridiculous.