Over the years, Mesis has worked for a number of celebrities and other noteworthy people. Photographs of him with celebrities, judges and government officials adorn a wall in his basement. He did work for the administration of George W. Bush. He's also collaborated with Scotland Yard. A bobby hat sits on a glass cabinet, a reminder of that case.

Mesis retains an impressive knowledge of the suitcase-sized camcorders he lugged around in the '80s. He used to build customized surveillance vans (price tag per van: $60,000), outfitted with an array of video monitors, controllers and extra battery sets for the equipment.

Back then, surveillance required close proximity to your subject and sitting for hours in vans or hunched down in cars. Clients hired private investigators to have a warm body watching a subject or a location.

These days Mesis operates mostly from his suburban home. On the ground floor, the house is fairly nondescript: comfortable couches on fluffy white carpet in the living room, porcelain figurines carefully arranged in glass cabinets. But Mesis and his wife, Rosemarie Testa-Mesis, once a paralegal but now a private investigator herself, have turned their 1,500-square-foot basement into a 21st-century spy cave.

One room is stocked with boxes of spy equipment that Mesis sells through his online shop, PI Gear. An archive of PI magazine, which the couple publishes, lines several shelves of a bookcase. Mesis is also helping to train a new generation of private investigators and keeping veterans up to date with sophisticated new tools and skills at conferences all across the U.S.

Nowadays, private investigators need to be able to find people through databases and use forensics software to peruse employees' browsing history. They're learning how to operate drone cameras to monitor the movements of a subject. Private investigators have to be more like Inspector Gadget and less like Dick Tracy. And Mesis is as tech-savvy as they come.

"The PI industry is changing dramatically," says Mesis.

"You're more likely to find a PI carrying an iPad than a gun."

Large information purveyors like Google and Facebook are increasingly under scrutiny for amassing data about their users. Edward Snowden's leak of NSA documents this summer shows how far the government is willing to spy on private citizens in the name of national security. But Mesis and private investigators like him represent a different breed of small-time snoopers with similarly technology-driven methods.

Recently, Mesis ran the names of a few hundred minors through a database to see whether their parents had committed residency fraud by sending their children to a school in a district that wasn't their home.

He regularly does background checks for employers, buying data for $20 to $25 from companies that aggregate information derived from Social Security numbers and addresses. In the past he would have had to interview neighbors, relatives and others to ensure he was tracking the right person.

But data-mining software isn't the only new tool in Mesis's arsenal.

GPS trackers used by law enforcement have dramatically dropped in price and have shrunken from the size of a shoe box to that of a nine-volt battery. Being able to track cars using GPS has made Mesis's job much safer. He no longer has to speed through stop signs and red lights or risk being discovered by an angry suspect.

Surveillance cameras have also gotten more convenient. Many now transmit footage wirelessly or can be attached to drones, allowing Mesis to monitor most subjects from a secure location far away from scenes of a potential crime.

Mesis does not miss the old days. He can now get much more work done in less time, thanks to his new tools.

These tools are also widely available to the general public. People who might have been Mesis's clients a few decades ago are now taking matters into their own hands. They often purchase the same gear that Mesis uses, sometimes from Mesis's online store. Empowered by cheaper and easier-to-use gear, they are now spying on their loved ones, but with less oversight than Mesis and other licensed PIs.

A professional PI runs the risk of losing his or her license, livelihood and reputation if audited for misconduct. Professionals know what they are and are not allowed to do, based on privacy laws in different states. But amateurs don't always do their homework.

"If you're being watched by a PI, you can be sure that it's done in a legal manner. It's what the public does that we should be worried about," Mesis said. "When people spy on their loved ones or employees, there's always an emotional qualifier that spurs their surveillance."

Based on the sheer numbers of websites and products available online, the consumer spy-gear market appears to be booming — from the latest GPS tracker with street-level mapping to smartphone spyware that allows you to read other people's text messages. Ratings vary from best designs, image resolutions, battery life, mounting brackets and USB data transferring functions. That's where experts like Mesis and Jason Lazarus come in, to help their clients wade through the market supply.

Lazarus is the owner of the website GadgetsandGear.com, based in New York City. He sells an array of spy gear in addition to wonky holiday gifts and office-desk toys. He has been selling spy equipment for nearly four years and says the three most popular items are surveillance cameras hidden inside ordinary objects, GPS-tracking devices for cars and mobile-phone-spying software, which allows people to extract anything from GPS data to e-mails and call logs from phones used by other people.

Although private investigators, law-enforcement officials and government employees purchase gear from Lazarus's store, most of his customers are spouses or lovers who want to spy on their partners. There are more practical uses to which his clientele put the equipment. Farmers, for instance, place GPS trackers on cattle to help find a herd when it gets lost. People who want to mail valuables like jewelry use them to monitor the progress of packages. Parents use GPS trackers to keep a tag on their teenage children.

People even buy countersurveillance gear to find out if someone is spying on them.

More than 100,000 of Lazarus's orders each month relate to surveillance and security. He says the number of customers buying spy gear has roughly tripled every year since he started his business.

"There's not a day that goes by when we don't get a call about spy gear," he said. "They want them immediately and they want them now."

Many customers buy the equipment to alleviate a "panic" they feel, Lazarus explained. They want to answer nagging questions: Is he cheating on me? Is someone stealing my mail? Is my child driving my car above the speed limit? Is my mother getting home safely?

"We're providing the type of products that help people relax," Lazarus said. "It's all about the unknown. If I don't know what's happening, those fears grow."

Despite the vast array of spy gear now available to private investigators as well as ordinary citizens, it's not always clear from a legal standpoint what they are allowed to do with it. Laws differ from state to state and from device to device. Some laws apply to everyone — private citizens, PIs and law-enforcement officials. Others only affect specific segments of the spy-gear market. In some jurisdictions, private investigators have more freedom to retrieve certain types of information or to use certain types of equipment.

Lazarus has a rule of thumb: customers are usually allowed to spy on others so long as it happens on their own property or if the gear is planted on a device that they at least partially own. An employer, for instance, can install spy software on a company phone. A mother can track the movements of the family vehicle. Nanny cams can legally be placed in your child's bedroom.

"In the U.S., we use a sectoral approach to privacy," said Paul Stephens, director of policy and advocacy for Privacy Rights Clearinghouse, which seeks to educate people about privacy issues. Unlike in Canada and Europe, which have broad, overarching policies regarding privacy, he explains, in the U.S., "you need to look at very specific privacy laws that apply to different technologies to understand whether something you're doing is legal or not."

Typically, laws governing privacy are created in response to technological advances. In the U.S., this gets down to specific devices. It's almost like a game of Whac-A-Mole: a new technology pops up, someone uses it in ways that invade other people's privacy, people complain to their legislators, and a law specific to that technology and device is created.

The level of specificity of some of these laws is astonishing. One from 1988, for instance, prohibits video-rental stores from disclosing the viewing history of customers, although that law was recently amended to allow users of online streaming services like Netflix to share their viewing history on social media, if they choose to do so. Another law from the same year prohibits most employers from using lie detectors on employees. The Do-Not-Call Implementation Act of 2003 allows individuals to prevent telemarketing companies from contacting them.

At the same time, there are plenty of tools and gadgets that aren't specifically covered by laws, such as GPS trackers and the use of cookies to compile web-browsing histories of individuals. The legality of their use becomes subject to the interpretations of judges. Stevens said there is always a time lag between when a technology enters the market and when a law is put in place to protect the privacy infringed on by that technology.

Lee Tien, a legal expert from the Electronic Frontier Foundation, which advocates for online consumer privacy, says the specificity of these laws makes them more business-friendly than the blanket approach of Canada and the European Union. That's because a narrowly written law can only be applied in very specific circumstances, and with ever-changing technology, there are always passageways for those who might undermine lawmakers' intentions.

For instance, any phone conversation requires the two parties to use a service provider such as AT&T or Verizon, which in turn creates a record of that phone call within these companies. Making a purchase from a vendor on Amazon with your credit card creates three records of that transaction: one with Amazon, one with the vendor and one with the credit-card company.

"The real big growth has been between individuals and intermediaries," Tien said about the growth of data collection. "The way we do things creates more records."

According to a business report from the MIT Technology Review, a typical American office worker produces about 5,000 MB worth of data per day, merely by downloading movies, Word files, e-mails and by moving data around on the Internet and on mobile networks.