A/N: Let's call this a backstory of sorts for Kivrin's superb Modern AU "The Sons of Durin". Read that one first, otherwise it won't make much sense.
Looking back through the centuries, two things had always kept constant about the Sindarin.
One: they were all tall, slim and graceful. Sidhe blood, a less enlightened age had called it, or fey folk changelings; people had made hex signs behind their backs – and some still did.
They had refrained from doing worse due to the second continuity: ever since Dabíd mac Maíl Choluim, or David I, had established the sheriffdom of Edinburgh in the twelfth century, and offered a cadet branch of Telerin nobility the prestigious, if demanding, position of dealing out justice in his name, a member of the house had always upheld the King's Law in the Lothians. Kings and dynasties had changed throughout the turbulent history of Scotland, but the Sindarin had held onto their ancestral post, cementing a reputation for being merciless but fair.
The Heritable Jurisdictions Act in 1747 had thrown them, for a bit. But the law was the law, and if the office of Sheriff Principal was now largely ceremonial (and couldn't be passed on along a family line, anymore), justice still had to be served – and competently so! There had been Sindarin in Cambridge, studying jurisprudence, nearly from its founding; as such they were perfectly qualified to take charge of sheriff courts and people already expected them to administer judgement – things went on pretty much as before.
As soon as actual law enforcement, police work, became a respectable profession, especially the younger members of the ancient noble house flocked to that, too. There was something about the tracking, the hunt, the catch, that strongly appealed to something very old within them; ridiculously overqualified for their jobs as they might be, in some cases.
OoOoOoO
It had been Thranduil's grandfather Taenthin, returning from a distinguished service in WWII, that had impressed on him the values of well-trained, well-disciplined subordinates beyond his immediate staff, who spread their influence not only among the upper echelons of the police force, but the ranks as well. He joined Edinburgh City Police, made Superintendent before he's thirty – his service record and pedigree might have had something to do with that – and promptly set up mandatory courses for all the policemen under his command.
Officially, they were physical fitness trainings; de facto, his trainees learned a lot more. Within months, Taenthin's Boys were easy to spot in a crowd, by their fluid grace. Taenthin's Dance Troupe, people immediately scoffed, both colleagues of the law and those disregarding it, the ridicule quickly growing vicious. Few people dared to offer taunts straight to the Superintendent's face, and if they did, he merely raised an unimpressed eyebrow, bred for haughty confidence for countless generations as he was. His Boys, not so much. But they did have the coordination, the flowing moves and the very serious self-defence lessons, he had drilled into their bodies and minds for months, and the derision soon met an abrupt end.
Applicants from other divisions soon asked for training, too, and though it took years – decades in some particularly obstinate cases – to convince all of the other division heads, by the time Taenthin retired and his son Oropher took up the mantle, it was only the graceless newbies, that stood out.
Oropher spread the lessons to all law enforcement staff, including prison guards, and by the time Thranduil takes over, he can pretty much take things for granted.
In a way, he does. He has been raised to a strict black-and-white, lawless-and-lawful dichotomy, that holds little place for extenuating circumstances. Plus, he's used to having his family name recognized, and wherever it is known, it's essentially a synonym for bulldog adherence to the law; as such, only a fool would try to break it right under their noses – and Smaug can be called a lot of things, but never a fool.
The so-called "Sons of Durin", on the other hand… they certainly are fools, of the emotion-driven, never-look-before-you-leap kind, and obviously lawbreakers. Elrond, a distant relative of Thranduil, has led the initial raid, back then, and though they don't have much contact, usually (something about an absolutely unsuitable match, a few generations back, and nasty things were said, including: halfblood; but, well, it was a different age), there is no need to doubt the other Sindarin's take on events.
It takes an unassuming grocer showing unexpected vehemence – and Elrond questioning his own conclusions – for Thranduil to take a second look.
A/N 2: Taenthin is a name made up whole cloth, as the ancestry of Thranduil seems to be unknown, save for his father.