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"。 his 是代词 he 的所有格形式，表示"他的"。 nose 是名词，指"鼻子"。 under his nose 是一个英语习语，字面意思是"在他的鼻子底下"，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。