The evidence was hidden right under his nose the whole time, but he never noticed it.

under 是介词，表示"在...下面"或"在...之下"。 nose 是名词，指"鼻子"。 under one's nose 是一个英语习语，字面意思是"在某人的鼻子底下"，实际表示"就在某人面前/眼皮底下"，强调某事物非常明显却未被注意到。