The Darkest Evening, by Ann Cleeves

In the teeth of a blizzard, DCI Vera Stanhope of the Northumberland & City Police sets off for home but becomes disoriented in the tangle of snow-covered rural roads. For a while she is able to follow the lights of another car, but then they disappear, and she finds the car veered off onto a farm track, the driver’s door standing open and—upon investigation—a toddler strapped into a carseat.

Assuming the driver has gone for help, though surprised they left the child, Vera takes the child with ...