under one's nose直译“在某人鼻子底下”，其实真实含义是“在某人眼皮子底下，公然的”。

【例句】
I searched everywhere for the letter and it was under my nose all the time!
我到处找这封信，可它一直就在我面前。

The police didn't know the drugs ring was operating right under their noses.
警方不知道贩毒集团就在他们的眼皮底下运作。