I checked the various traps I always leave set for unwary intruders: single hairs gummed across cupboards, ends of paper sheets jammed haphazardly in drawer openings (on the assumption that no snoop would ever be able to resist straightening them), and, behind each of the inner doors, a thimble filled to the brim with a solution of insoluble ferrocyanide of iron, or Prussian blue, which, once spilled, could not be washed away if seven maids with seven mops swept it for half a year.
