There are 13,000 yellow cabs racing around New York City. So when James Dunne III  realized he had left his briefcase containing $5,000 in cash, personal checks and valuable company documents, he assumed he would never see it again.

But earlier that morning, Dunne had met Andrew Murstein, president of the Medallion Financial Group. It took Murstein less than an hour to figure out who Dunne's cabbie was.

And fortunately, he was an honest cabbie and returned Dunne's briefcase with everything in it.

Murstein's sleuthing on March 14 made use of a GPS system being built into the taxis.

Murstein, whose company helps in the servicing of loans for New York City yellow cabs, figured by using the GPS, the Taxi Tv in the cab's back seat and credit card system data, he has a "good chance" of figuring out which of the thousands of cabs was in a particular place at a particular time. Dunne's case was a little bit harder because he didn't pay with a credit card, Murstein told ABCNews.com.

For Dunne, an executive of the Sandler O'Neill financial firm, the briefcase was more important than the money in it.

"I knew right away about the money. I was curious why there was so much money, but I didn't want to ask," Murstein said. "He told me it wasn't about the money, but about the other items."

Murstein said people are always looking for help in tracking down taxi cabs because they left something behind.

"I get calls for all kinds of items. Everything from 'a really ugly shirt that my mother-in-law bought me that I have to wear tonight when I see her for dinner' to 'a priceless picture of my grandparents.'   The most frequent item these days are iPhones or blackberrys. People seem to not be able to survive without them," he said.

The GPS systems in the cabs originally threatened to be controversial when the first ones were installed in 2007, with drivers fearing that their bosses or the city's Taxi and Limousine Commission would use them to keep track of them.

Murstein conceded that it could be used like that. "The GPS can be used to make sure a cab is going in the right direction and not giving a tourist a run around. It tracks the entire cab route," he said.

However, Fernando Mateo, the New York State Federation of Taxi Drivers spokesperson,  said the systems have provided a sense of safety for both passenger and driver.

Mateo said the systems have been used to aid police during armed robberies and provided taxi drivers with an extra sense of security.

"This is exactly what we wanted," Mateo told ABCNews.com. "Whenever you're serving the public, people should feel safe when entering the cabs."

Mateo says that in addition to briefcases, he has heard of violins and jewels being returned to their owners after being tracked down through the GPS system.

"When people come to New York, they know their public transportation is safe," said Mateo, who said the New York State Federation of Taxi Drivers, had supported the use of the GPS systems. "And if anything happens, we can track it."