Look at the nearby post "you should be able find your thing" instead of "be able to find", a word has been dropped. In the same way of casual speech, the subject of dunno is sometimes left out. "Dunno" for "I dunno". About your question, I can't tell anything but what I can imagine. Ex. - Will it be raining this afternoon? - Dunno. - You dunno? Though you got the weather forewast.