For the prompt: apply your favorite TV Trope here.
I chose dissonant serenity.
By the time it's found, the body has begin to rot. The skin is blackened and pitted, the stomach bloated. It eyes crawl white.
Even John, who has lived amongst death, turns away. He can hear Lestrade swallowing heavily beside him, no doubt trying to keep himself from vomiting.
The window is curtained with flies - glittering, undulating sheets that block out the light. It's almost beautiful. Almost.
It's the stuff of nightmares, really. Already, he knows that is one of those things that will keep him up at night, twisting and shaking and trying to forget. People haven't stopped doing horrible things to one another simply because he's entered civilian life.
The smell. The smell is beyond description.
"Wh-" he starts, and inhales hot air. Decomposition makes the room warm. He swallows and tries again. "What happened?"
There is a blur beside him. He turns, and Sherlock is kneeling beside the corpse. For once, he has deigned to wear a biohazard suit, less to prevent contamination of evidence than because the location is likely to be loaded with infection. He is wearing clear plastic gloves. John watches him flip over the victim's wrists, watches him press his fingers to flesh that falls inwards at a touch.
Within the minute, Sherlock stands. He tips his head, turns back to the corpse for a brief moment, then looks at Lestrade. The man pulls out a notebook.
"Revenge killing. Rings on all the fingers except the ring finger, which means someone's taken it away. There are pictures of family and friends in the room, but the body's been here for quite a while, so she had acquaintances but lost them. He's abusive, then, emotionally and - judging from the lacerations on her wrists - physically. We're looking for a man approximately five years older than her, financially stable, with a history of violence, and petty crime."
He gives John the little smile which means he's right, then pulls off his clear gloves and tosses them on the ground. "Perfectly mundane, open-and-shut, call me when you find the killer."
He turns, and walks out. He doesn't rush. He merely moves on, placidly, because he's got better places to be.
John watches him until he leaves the room.
Then he shuts his eyes.
He can hear the buzzing of the flies.