No one in the evening crowd at a Starbucks knew Humphrey Cheung. But he knew things about them. Several tables away was a guy sitting alone with his own laptop. “He’s starting a business,” Cheung said. And the young couple in the far corner? “They’re getting married,” he confided.

Cheung isn’t psychic. He had hacked into the coffee shop’s wireless Internet connection on his Toshiba laptop. It took about five minutes to do so, using free software available online.

Public Wi-Fi, or “wireless fidelity,” is very handy for perusing the Net away from the office or home. Just remember that you may have company while surfing. “When people are on a public wireless connection, they have the same expectations about privacy as when they are on the Internet at home,” said Cheung, 32, a computer security expert and an editor for TG Daily, a technology news website. “But it doesn’t work that way. Someone could be listening in.”

Cheung was using a “sniffer” program that intercepted online signals as they flew back and forth from the laptops to a wireless modem hidden somewhere amid the coffee paraphernalia. Mostly, the monitoring was limited to tracking the websites being visited. Numbers correlating to web addresses flew across Cheung’s computer screen, allowing him to see that the couple was viewing pages at a wedding-planning site. The man a few tables away started with sites selling high-speed broadband service, then went to a page about managing websites. Like a mystery yarn, the clues kept coming in.

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