When a high-ranking official in China’s public security system was elected president of Interpol in 2016, leaders in Beijing rejoiced. The promotion lent respectability to China’s notoriously opaque and arbitrary criminal justice system. But now that same official, Meng Hongwei, 64, has himself mysteriously disappeared, after recently returning to China. Even the country’s most internationally prominent police officer, it seems, can vanish without an official murmur from Beijing. No one seems to know where Mr. Meng is or why he suddenly disappeared, even though he leads an organization that serves as a kind of United Nations for the world’s police forces.