New answers tagged subjects
It's because it's the most straightforward choice. Using は would sound as if you are wondering if there's at least one person who knows it among people present there, and using が would sound as if you are wondering if there's a person in the world who knows it. Incidentally, the topic of the whole sentence is 誰が盗んだのか, (edit) that's why it's not marked with ...
Both が and か could be used after 誰. However their functions are different as follows: が indicates 'sentence subject'. "誰が盗んだのか" literally translates to "About who stole (something)". か also indicates 'sentence subject'. However, it has connotation of uncertainty and doubt. "誰か知りませんか" literally translates to "(I am not sure whether somebody could find out ...
I don't know 誰か is used as topic but 誰か知りませんか? is common and I think that is grammatically correct. It is translated as "Does anyone know ～?" and your sentence is "Does anyone know who stole?".
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