疑问代词表示虚指

Interrogative pronouns of indefinite denotation

Besides forming questions or rhetorical questions, interrogative pronouns can also be used to denote a person, thing, time, location, or manner that is unknown or uncertain to the speaker, or that the speaker is unable or unwilling to tell. For example,

你应该吃点儿什么。
我不记得放在哪儿了。
这件事儿好像谁告诉过我。
我不知道怎么扭了一下胳膊。