Coffin's Dark Number

the long version

Three little girls had been reported missing recently in the same district of South London and Superintendent John Coffin was aware there could be others. The area had more than its fair share of cranks. For a start there was Tony Young's club of UFO watchers. Tony himself was worried by the fact that each time a child disappeared, members of the club were out investigating a sighting. What could be going on? While Coffin probed the backgrounds of the victims, trying to establish a pattern, Tony did his own detecting. And, in the end, it was Tony who supplied the answer, as obvious as it was unexpected.