[nouaru-taka-wa-tsume-wo-kakusu]Literal translation:A competent hawk hides its talons. Meaning:A competent or talented person usually holds back from showing his/her true ability, and exerts it only when necessary. 

能ある鷹は爪を隠す

[me-kara-uroko-ga-ochiru]Literal translation:Scales fall from the eyes.Meaning:Have scales fall from (one's) eyes (English idiom)Suddenly see a situation clearly and accuratelyNote:This idiom is shared by the Japanese and English languages.

ほっぺが落ちる