Finwë could flap his ears, like not a bit but full on angry cat style flap them down when he was sad. That was something absolutely unique to him that none of his children or other relatives could do.
He had the flapping ears, but neither Elwing or Eärendil knew what it was so it was just labeled as a weird quirk, he had many of those.
It never really came up again.
Until the third kinslaying.
The whole battle happens, Elwing flees with the Silmaril, Elrond and Elros get kidnapped and taken hostage by Maedhros and Maglor, the whole story.
They get taken to Amon Ereb and everything seems normal, just two half elven kids with some ainuric features to themselves, nothing too weird.
But one day Maedhros sees.
One day Elrond accidentally ran into a podium and pushed off the vase that stood on there, shattering it completely. He of course apologized immediately, cause it’s Elrond, of course he’s polite, even as a child.
Maedhros assures him it’s fine and not to worry and cleans the vase off the floor so no one gets hurt. Afterwards he goes to Elrond to reassure him he’s not angry because the boy seemed startled.
He walks into the room Elrond and Elros slept in and looks around for him. Elrond sits on the bed and turns to Maedhros, who them sat down at the foot of said bed and made sure Elrond knew he wasn’t angry.
Elrond however apologizes again anyways and this time his ears flap because he feels guilty.
And Maedhros‘ mind goes absolutely blank.
He gets thrown back in time and immediately has his grandfather in front of his eyes, it doesn’t help the case at all when Elrond looks up at him and his ears go even deeper because he thinks he somehow upset him.
Maedhros stumbles back, half convinced he’s hallucinating, but after a few moments he realizes he’s not, and he breaks down.
He simply collapses onto the floor and begins sobbing. Big, heavy, crocodile tears. His entire form is trembling, his eyes red, and his mind in going into overload with memories.
Moments later Maglor walks in, having heard the commotion. At first he’s confused at seeing his big brother on the floor crying, but then he sees Elrond, and time stops.
He doesn’t break down, he doesn’t cry or shake. He just freezes, his limbs simply refuse to move and make him freeze in place like a statue.
They stayed like that for some time until Elrond began to sniffle, thinking he did something wrong.
They explain it to him and he feels really proud for having something in common with such a great elf.
From that day on Maedhros treats Elrond like a son. He can’t stop himself, it just feels wrong not to. He clings onto that small detail on the child that was supposed to be his hostage and it helps him, it help him so much.
Elrond, who by now understood the reason behind their reactions, starts using it as a way to help them. Whenever he spots either Maedhros or Maglor sad he goes up to them, climbs onto their lap and flaps his ears, and they hug him, and it helps.