It isn't, because word order is very strict in Chinese, and the subject of the sentcence (in this case 谁/who) must always come before the predicate (in this case 在/to be somewhere). It's true that 在 has several different meanings / uses in Chinese, but here it indicates existence at a certain place (with the structure: 在+place), and that place cannot be 'who'.

I thought the same thing

Replace 在 (be in/at/on/…) with 是 (be) and you will be correct.