在哪儿 / 什么
zài nǎr / shén me
Expressing "some" with question words
A question word without a question means "some" — somewhere, something, someone.
Question Word used indefinitely
How it works
这个人我在哪儿见过 — I've seen this person somewhere. It isn't asking where.
Intonation and context do the disambiguating: a question rises, a statement doesn't.
Common in vague statements: 我想吃点什么 — I fancy eating something.
Examples
zhè ge rén wǒ zài nǎr jiàn guò
这个人我在哪儿见过。
I've seen this person somewhere before.
wǒ xiǎng chī diǎn shén me
我想吃点什么。
I feel like eating something.
hǎo xiàng shéi zài qiāo mén
好像谁在敲门。
It sounds like someone's knocking.