Can ne zaman come before hırsız in this sentence?
yes you can. but the element that is just before the verb is emphasized in sentences. so if you ask a question it's more clear to put question words just before the verb.
İf "hırsız" comes before "ne zaman" would be more accurate.
If it was bulunuyor what would the sentence mean?
If you say "bulunuyor", It would be absurd, "when is the thief being found?", as to say such a sentence.
Oh ok that seems obvious now. Thanks :)