FILE- This May 2, 2012, file photo shows a sign advertising job openings outside a McDonalds restaurant in Chesterland, Ohio. McDonald's said a key sales figure climbed 3.7 percent in August, as the fast-food chain emphasized the value of its menu offerings amid the challenging global economy (AP Photo/Amy Sancetta, File)

Before she got fed up and quit last month, it wasn't uncommon for Darenisha Mills to keep working after her shift ended at the McDonald's in Pontiac, Mich., where she was a cashier.

"They're asking you to clean the bathrooms, sweep the lobby, run the register," the 26-year-old told The Huffington Post, "but they don't pay you anything for the time you work over."

The formal name for that is wage theft, which occurs when an employer withholds pay rightfully earned by an hourly worker. It happens in a variety of ways, from not paying for overtime, to denying mandated breaks, to subtracting hours from employees' weekly total.

A recent poll commissioned by labor group Fast Food Forward estimates that a stunning 89 percent of fast-food workers have experienced at least one form of wage theft. A previous study, conducted in the first half of 2008 before the recession, found 68 percent of low-wage workers had been victims of wage theft in their previous work week, and estimated that wage theft cost workers an average of $2,634 annually.

"The survey [from Fast Food Forward] lays bare the fact that wage theft is rampant," said Tsedeye Gebreselassie, an attorney with the National Employment Law Project, which advocates for low-wage workers and performed the 2008 survey. "It's pervasive throughout our economy."

"They want to keep labor costs very low," said Kwanza Brooks, 37, who was a McDonald's manager for over a decade in Maryland and North Carolina before quitting a couple years ago. "Taking the wages was the only way they could control it," says Brooks, who now volunteers for Fast Food Forward in Charlotte.

The Fast Food Forward poll found that 84 percent of McDonald's workers who responded had experienced wage theft. Hart Research conducted the online survey between Feb. 15 and March 19 on behalf of "Low Pay Is Not OK," a campaign affiliated with Fast Food Forward. The poll surveyed 1,088 fast food employees, including workers at Wendy's and Burger King, in the top 10 metro areas nationally.

McDonald's cautioned against drawing broad conclusions from the survey. In a statement the company called it a "small, random informal sampling." The company said it believed workers should be paid correctly.

McDonald's and its franchisees are now facing six lawsuits in three states, involving tens of thousands of employees, claiming various wage theft violations.

Wage theft can become increasingly common in times of high unemployment, experts say. "When people are desperate for jobs, they're afraid to risk them by taking on their boss," said Ross Eisenbrey, of the Economic Policy Institute, a left-leaning think tank.

And because the amounts of wages being withheld are often small, it can be hard for a low-wage worker to find an attorney willing to take their case.

For their part, fast-food managers are under "tremendous pressure" to keep labor costs low, especially when sales are sluggish, said Nelson Lichtenstein, the director of the Center for the Study of Work, Labor, and Democracy at the University of California Santa Barbara.

Companies may also engage in the practice when the risk of getting caught is low. There aren't nearly enough U.S. Department of Labor investigators to enforce the laws, said Gebreselassie. He added, "The chance any worksite will be investigated is miniscule."

Nevertheless, the issue of wage theft has been getting increased attention in recent months. In March, the owner of seven McDonald's restaurants in New York was ordered to pay almost $500,000 to more than 1,000 employees who performed work off the clock and had other pay illegally withheld.