The phone not only disclosed her location, it also recorded 99 hours of conversations.
Later, she learned that he'd also installed spyware on her computer that revealed to him all of her keystrokes, browsing history and e-mail conversations. Because of that software, he was able to read correspondence with her lawyer and obtain passwords to her bank accounts.
Sherri's estranged husband was arrested on charges of felony stalking in 2006 and eventually sentenced to eight months in jail. But despite the relief she felt when she learned just how he had been stalking her, she said the fear has never lifted.
"It's like having your own personal terrorist -- the fact that a person can violate you over and over..." Sherri said. "We use technology for work, for personal use, in every aspect of our lives. Once you've been violated with technology you never trust it again."
A Department of Justice report released in January said that about 25 percent of the 3.4 million stalking victims in the United States reported cyber-stalking, such as e-mail or instant messaging. GPS technology and other forms of electronic monitoring were used to stalk one in 13 victims.
Safety and privacy experts say that as smaller and cheaper technology and social networking sites proliferate, it is becoming easier for stalkers to monitor the whereabouts and activities of their victims.
"The majority of the technology does have legitimate purposes. ... The problem is not with the technology itself but with how it's being used," said Michelle Garcia, director of the National Center for Victims of Crime's Stalking Resource Center.
And as an increasing number of people misuse that technology, Garcia and other advocates encourage those who think they are victims of technology-enabled stalking to trust their instincts and seek out advocates at organizations such as the National Center for Victims of Crime and the National Network to End Domestic Violence's Safety Net Project.
They also suggest using safer computers at libraries and cafés and getting new phones with numbers known only to a few trusted individuals.
They also urge those who would use technology to spy on a partner to consider other options.
"When you start getting into the issue of tracking your spouse, you can't cross that line into the realm of stalking. Context is everything," Garcia said. "Someone can cross that line into where they are committing a crime and may not even know it."