Replace the dashes with commas.
The verb in the third sentence belongs with the noun clue (and even if you think it belongs with clothing, that’s not a plural noun, either): clue is, not clue are.
Missing person cases are beginning to form clusters in mountainous regions around the globe. People, mostly children, are vanishing into oblivion under unexplainable circumstances. Sometimes the only clue left behind is their clothing, neatly piled.