Murder Riddles

A Midnight Murder

Mr Turner calls to his wife, who is busy doing the washing up, and he asks , "Darling? Why don't we go for Midnight drive? The kids aren't home..." He said, giving his wife a suggestive look, then adding, "and we haven't done something like this for ages!"
"Well, why not?" the woman said, abandoning the dirty dishes and going upstairs to wash her face and put some different clothes on. But as soon as she was out of sight and the husband was sure she was gone, Mr Turner picked up the silver knife he had been cleaning, and smiled at his reflection...

The husband got into the car first, slipping the murder weapon into the cubby hole. His wife joined him a few minutes later, wearing a pretty flowery skirt, a blue blouse, her brown locks falling like a waterfall over shoulders. Mr Turner quickly straightened up, his grip tightened around the steering wheel. He pushed his foot on the accelerator, and they sped off.
They went down winding roads, passing trees, and cute cottages. They went down the path they had travelled a lot when they were first married, as the wife had pointed out multiple times.
After while the husband pressed his foot down on the breaks, stopping near a cliff face, where they both looked up at the dark sky.
"Remember Harry, this is where we shared our first kiss!" Mrs Turner whispered to her husband, leaning into him.
They looked up at the winking stars, the last thing the wife would ever see. Suddenly, the husband reached across his wife, causing her body to lean back onto the seat, while she muttered cries of confusion. He wrenched the cubby hole open, seized the knife, and plunged it into her chest. Her cries didn't stop him from stabbing her over, and over again. Images flew before him every time he felt her wince and scream. The other men in his bed, their arms around his wife's neck. There were too many for him to forgive her. He only stopped, when his wife's eyes had become lifeless, and her body stiff, the blood seeping through her blue blouse, staining the material. He looked at his wife's lifeless body, but he felt no guilt. But he had to act quickly.

He grabbed her body, shoving it out the car, making sure not to leave any fingerprints. He then grabbed the knife, threw  it over the cliff face, never to be found again. He then jumped in the car, and drove  away in the tar road, making sure he didn't leave any tracks, back to his empty house. 

He got there, his footsteps echoing down the passage, where his wife had walked before. He walked into the lounge, jumping at every noise.He almost screamed when the phone rang about two hours later. He ran towards the phone, picked it up, hoping that his voice didn't sound suspicious. It was the police station, informing him of his wife's passing, and that he should come to the crime scene immediately. 

He slammed down the phone, and hopped into the car, with which he had earlier driven his wife to her death.
When he arrived, he was greeted, not with pity, condolences or forgiveness, but with the clanking of metal, which came from the Detective Inspector slipping handcuffs around Mr Turners hands.

How did they know he was the killer?
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