Disclaimer: I do not own BBC Sherlock or any other related ACD works :(
Greg Lestrade stared at the strange man standing in front of him. Having just been made Detective Inspector with Scotland Yard, Lestrade was by no means unintelligent. But this man had literally walked into a crime scene, quite out of the blue, examined the body for less than 10 seconds and calmly told him that the murder Lestrade was investigating was committed by a 5'8 female doctor, with straight red hair and nails painted green.
'And how on earth do you know that!?' Lestrade asked dumbstruck. The man frowned. 'Why, it's obvious, isn't it?' With no reply but a confused stare, he sighed. 'Oh, how simple your minds think.' Lestrade was getting angry now.
'Look, if you have nothing useful to report in relation to this crime, I will have to ask you to leave.' The man rolled his eyes.
'Very well. The victim is a petite blonde woman, married happily for less than two years. Hmm, not a marriage on the rocks, like yours' is. So she was faithful, unlikely to have had an affair, so that is not the motive. She was choked around the neck with what appears to be a stethoscope. The murder weapon is consistent with the stethoscope you just had taken into evidence. That implies doctor, not to mention this area is off limits to patients. So, she was strangled by a doctor. Doctors have to wear flat shoes in hospitals. The angle at which she was choked suggests the killer was around 5'8. The red hair caught in her necklace that you obviously missed is long, straight and red, clearly belonging to a woman. The hair is naturally red and straight, with no obvious signs of excessive heat damage or dye. The victim's nails are blue, so this chipped green nail polish left in the scratches in our victim's shoulder must belong to our assailant. Likely she scrawled her during the struggle while she was trying to kill our victim. Did you get all that?' The man tapped his foot impatiently.
'W-wait a minute!' Lestrade stuttered. 'Back up. How do you know she was happily married for less than two years? And for that matter, how do you know that my marriage is in trouble?'
'Her ring!' the man exclaimed. 'Her ring?' 'Yes, her ring! It's designer, only been in business for, yes two years. It's spotless, well cared for her. State of her marriage right there. As for your marriage, you have been sleeping poorly for the last few nights, evident in your constant yawning and bags under your eyes, not to mention you keep rubbing your back. You could have got a new mattress but unlikely. So you've been sleeping on a couch. Now doesn't that just scream marital bliss?'
Lestrade narrowed his eyes. 'If you're making this up off the top of your head.' The man sighed again. 'Does it appear I'm wrong?' Lestrade glared for a minute.
'Wait here for a minute.' The man shook his head. 'Impossible. I have a skull to collect from the mortuary at St. Bart's. However, you can text me at this number.' He dictated his number as he fixed his scarf. Lestrade scribbled it down furiously.
The man began to walk away quickly, his long coat flapping around him like an evil villain. 'Wait, I didn't catch your name!' Lestrade called after him. The man stopped for a minute, turned and smiled slightly.#
'The name is Sherlock Holmes.'
Just a little one-shot to satisfy the little plot bunnies