Mr. Policeman and the Cook

A man is murdered and his wife suspected of the crime. She herself believes that she committed the crime while sleepwalking. She is cleared, but the crime remains unsolved, until many years later the investigating policeman confesses that he allowed the real culprit to escape.

Sensational non-supernatural crime story; the supposed murder while sleepwalking qualifies as an element of the fantastic, although it is a red herring rather than an actuality.
"Published as 'Who Killed Zebedee?' in The Seaside Library (23 January 1881); retitled 'Mr Policeman and the Cook' for its republication in Little Novels (1887)." — information from p 383 of Norman Page's collection of Wilkie Collins short fiction Mad Monkton and Other Stories.