The discovery that an Austrian man held his daughter captive for 24 years is shocking. Yet more abhorrent is his admission to an incestuous relationship with her, from which she produced seven children, their ages ranging from five to 19. The man's wife, who also had seven of his children, claims she knew nothing of the disgusting perversion taking place two floors below their home. Yet how were the sudden appearance of his daughter's children explained away to her?

This episode follows the 2006, finding when Natascha Kampusch, also Austrian, emerged after eight years in captivity in a Vienna suburb. But Austria alone is not to blame. In the UK, no one suspected Fred and Rosemary West when they killed and buried 12 victims beneath their house in Gloucester over five years, from 1973; while in Belgium Marc Dutroux held six schoolgirls captive in his cellar between 1995 and 1996.

It is too easy to ask why neighbours or relatives or social services weren't aware of what was going on, for if the perpetrators kept it secret, then there is little society can do until a chance discovery is made. Perhaps it behooves us all to be more vigilant to the possibility of such horrors occurring.