When the stolen items were mentioned, Sylvia immediately realized that Dirk was definitely not the thief. None of those items were at his camp. So this was a different thief this time around. Thinking about what was stolen and looking at Shortfang, she asked out of curiosity, "Hmmm...Were all of those things stolen at the same time?" 

She hoped that was the case, because if so, then the kobold's stature would be evidence in of itself. She highly doubted that a kobold would be able to carry a spear and a shield at the same time without making noise. 

If it turns out the three items were stolen at different times, then she would have to figure out a different way to prove Shortfang's innocence. 
She could mention the witness who saw the thief...