For a year, he just didn't seem the same. He wasn't where he said he'd be. He hinted that he was leaving. He said he wanted a break.
So, Michelle (who spoke with ABCNews.com on the condition of anonymity) spent $1,500 on a GPS tracking system to test her theory that her husband was having an affair.
The technology confirmed Michelle's mounting suspicion: Her husband of 14 years was seeing another woman.
"Spying could seem extreme [but] once you get to that point, you already know. It's more of a closure thing," she said. "I was blaming myself. I was turning into a state of depression. ... I know now that he's the jerk."
As surveillance technology, such as GPS tracking devices and video cameras, has evolved to become smaller and cheaper, more and more people like Michelle are turning to spy gadgetry to not just monitor their property, but the people in their lives.
But, experts warn that it's easy to cross the line. Stalking is illegal, and depending on your state, you could find yourself running afoul of the law without even knowing it.
Commercial-Grade GPS Tech Gets Personal
Earlier this month, the New York Post reported that former football player Michael Strahan installed a tracking device on his girlfriend's car when he suspected that she was cheating on him.
Nicole Murphy, the ex-Giant's girlfriend, reportedly discovered the device behind the front dashboard when she took her Range Rover to the dealership.
But people in the industry say Strahan is hardly alone in using sophisticated spy technology for personal purposes.
"Even in this economy, our business continues to grow," George Karonis, president of LiveViewGPS, told ABCNews.com.
Karonis sells a wide range of commercial-grade GPS tracking systems online and, though he would not disclose details, said "sales are brisk."
Most of the company's clients are businesses that use the devices to track delivery trucks or other mobile employees, but he said 40 percent of customers are individuals who want to recover stolen vehicles or monitor teen drivers, older family members or cheating spouses.
The company requires that all clients sign an agreement stating that they do not intend to use the technology for illegal purposes, such as stalking, Karonis said.
But he acknowledged that once the technology is out of the company's hands and activated, it can't monitor what its clients do.
"Truth is what people seek and are willing to pay any price to find -- or a reasonable price," Karonis said. This is "something that verifies the truth."
Technology Saves Companies Time, Money
U.S. Fleet Tracking supplies LiveViewGPS.com with some of its products and its president, Jerry Hunter, said business has multiplied by nearly a factor of two every month since he founded the company five years ago.
"We're the largest real-time tracking company in the world," he said. "Our products are used from everyone from electrical contractors to parents tracking teenagers and husbands tracking cheating wives."
This year, for the third consecutive time, U.S. Fleet Tracking monitored all team buses and VIP limos for the Super Bowl, he said.
By just looking at a computer screen, businesses can tell if their employees are where they are supposed to be and the technology has helped clients save time and cut fuel costs, he said.
Even a device no larger than a cell phone can allow another person to remotely monitor a car or truck's location in real time. Hunter said the system can be set to update locations once every one, five or 10 seconds and is accurate up to eight inches and a quarter of a mile per hour.
But he said that although most of their customers are businesses, they often fill orders for individuals eager to catch a cheating spouse.
Once, he said, his company received two orders from the same address on consecutive days. It turned out that both a husband and his wife suspected the other of cheating and unknowingly had two devices installed on the same car.
'StealthCams,' 'SpyHawks' and More
But other companies offer prying partners even more options.
An aspiring 007's dream, SpyAssociates.com markets everything from a "SpyHawk" real-time GPS tracking system and "StealthCam" alarm clock that hides a video camera, to a nearly $2,000 counter surveillance professional package.
"Greed, lust and fear are the three high-growth industries and this covers all three," said owner Jeffrey Jurist, adding that his clients include law enforcement, individuals, private investigators and corporations. "Everybody's watching everybody. It's just a matter of whether you're aware of it."
While the most popular items are currently GPS tracking systems and covert hidden cameras, he also sells listening devices and computer software that secretly saves keystrokes, Web site histories and e-mail messages.
When Hi-Tech Sleuthing Becomes Technology-Enabled Stalking
SpyAssociates also sells counter surveillance technology, such as bugging device detectors, transmitting video camera detectors and audio jammers that counteract listening devices.
But Jurist acknowledged that people who use this technology need to pay attention to the law.
"There's a fine line depending on how [it's being used] and who is using. It's up to the end user," he said, emphasizing that the law differs from state to state.
Jurist and others who sell spy tools say their cheaper, smaller devices let just about anyone with a modest budget accomplish anything they want.
But some of the targets of that technology maintain that it's an unfair, and often fearful, reality that they make possible.
"I feel it's unfortunate that a lot of these companies promote the technology for the use of tracking your spouse or finding a cheater, because what they're ultimately saying to someone is that it's OK to do this," Sherri, 39, a victim of technology-enabled stalking, told ABCNews.com. Sherri asked that her last name be withheld to protect her identity.
After Sherri tried to divorce her abusive husband of 10 years, she said he started appearing in random places at random times.
Three days after quietly changing jobs, he showed up in the parking lot of her new office. A month later, on her way to the airport, she spotted his car behind her.
"Things started happening that were not coincidental," she said. "I knew immediately -- right away -- that he was monitoring my movements, my whereabouts [and] my activities."
After six months, frightened for herself and those around her, Sherri sought out a victim's advocate who helped her see the patterns in her estranged husband's actions.
Ultimately, with the help of a detective, Sherri discovered that her stalker had installed a GPS-enabled cell phone near the radio in her car's dashboard.
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.
One-Quarter of Stalking Victims Report Cyber-Stalking
"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."