Trust Your Eyes by Linwood Barclay

A man is moving down a street on Manhattan's Lower East Side, scanning the storefronts, taking in his surroundings, when he sees something in a third-floor tenement window. A woman's face, distorted with terror. Thomas is convinced he's witnessing a murder.
Except that things aren't quite as simple as that. To some people, Thomas Kilbride wouldn't exactly be the most reliable of witnesses, and it's going to be very difficult to persuade the police to investigate.
Because Thomas was never actually there. And the murder was months ago.
But Thomas trusts his eyes.
And that could be his biggest mistake.