The missing letter had been misfiled.

找不着的那封信放错地方了。

《牛津词典》

The missing letter was in his pocket, if you please.

你看怪不怪，那封遗失的信竟在他的口袋里！

youdao

It was just an accident that I found the missing letter.

我找到了那封丢失了的信，完全是偶然的。

youdao

By skipping a single missing letter, the drug makes a shorter but effective version of dystrophin.

FORBES: In A First, An Experimental Drug May Help Boys With Muscular Dystrophy

He noted that Leahy's name was missing on the letter.

CNN: FBI director appears to contradict Gonzales' testimony

McClellan said the Iraqi government reported the missing weaponry to the IAEA in a letter dated October 10, and the IAEA informed the U.S. mission in Vienna on October 15.

CNN: Tons of Iraq explosives missing