Leslie James Pickering noticed something odd in his mail last September: a handwritten card, apparently delivered by mistake, with instructions for postal workers to pay special attention to the letters and packages sent to his home. “Show all mail to supv” — supervisor — “for copying prior to going out on the street,” read the card. It included Mr. Pickering’s name, address and the type of mail that needed to be monitored. The word “confidential” was highlighted in green. “It was a bit of a shock to see it,” said Mr. Pickering, who with his wife owns a small bookstore...