A button had come off my coat.

我的外套掉了一颗纽扣。

《牛津词典》

His shirts were always ragged, which caused him to button his coat to the chin.

他的衬衫经常是破了的，因此他总把短外衣一直扣到下巴。

youdao

A button has come off my coat.

我的上衣掉了一个扣子。

《新英汉大辞典》

So when in doubt, button your coat.

FORBES: To Button Or Not To Button, That Is The Question -Media Training

Like Q, Mendez has a collection of gadgets -- among them, a skin mole that carries a piece of espionage equipment called a bullet lens, and a coat on which one button is the lens to a concealed camera.

CNN: Books - Former CIA agent unveils secrets that made him 'Master of Disguise'

He saw us now and paused, one hand gripping a button at the front of his coat.

NEWYORKER: Midnight in Dostoevsky