Passing through a pair of metal blast doors, the woman—22-year-old Sheng Jiahui, who goes by the nickname "Sammy"—moves deep into dimly lit corridors.

Earlier this year, a 4-year-old Minneapolis boy died of lead poisoning after swallowing a metal charm that came as a gift with a pair of Reebok sneakers.

I have two devices to me solve mysteries like that: a camera with a macro lens, and a pair of surgeon's glasses, which are heavy metal frames with little binocular telescopes mounted on them.