SLEEPING with a work colleague is a dangerous business — and perhaps never more so than when your employer is Wal-Mart, the world’s biggest retailer.

Last December, the Arkansas-based company ousted Julie Roehm, a top-flight advertising executive the company had recently hired, and her junior colleague Sean Womack. At the time the company refused to comment on the reasons for the departure, but rumours began to circulate that the pair had been involved in an office affair.

In January, Roehm hit back, suing Wal-Mart for wrongful dismissal while denying claims put to her by the company that she and Womack had been involved in “improper behaviour”.

As critics and competitors have found, attack is Wal-Mart’s favoured defence. The retailer has come out swinging, setting out in lavish detail the evidence it has gathered on Roehm and Womack’s alleged improprieties — and in the process has lifted the lid on its fearsome internal police unit.

Employees gone bad can cause companies headaches, and Wal-Mart is not the only firm that investigates its workers to guard against fraud and protect trade secrets.

Nor is Wal-Mart alone in using what some would argue are heavy-handed tactics. Last year, Hewlett-Packard executives ended up facing criminal charges over action they took to track down boardroom leaks.

But few companies are as ruthless, or as thorough, in monitoring their staff as Wal-Mart. America’s largest private employer has a 400-strong team of snoopers led by Kenneth Senser, a former senior official at the FBI and the CIA. His deputies all hail from the upper ranks of American law enforcement.

Wal-Mart’s counter-suit filed earlier this month sets out the company’s policy: “Corporate executives are held to an especially high standard compared with other employees — and for good reason. They make business decisions that affect the lives and well-being of employees and shareholders. Their actions shape the future of the company, its image and its dealings with the public, customers and contractors.”

Wal-Mart goes on to say that Roehm “put her own interests first” and “flouted express Wal-Mart policies, not to mention basic principles of corporate ethics”.

The company accused Roehm of using company-paid travel to conduct an affair with Womack, of accepting meals and a case of vodka from companies competing for its advertising business, and pursuing a job with one bidder.

Roehm started work at Wal-Mart’s Benton-ville, Arkansas, office in January 2006 where the married Womack was already employed on a contract basis.

Wal-Mart investigators subpoenaed Womack’s wife, Shelley, compelling her to give sworn testimony about how she discovered a sexual relationship between her husband and Roehm and to turn over dozens of embarrassing e-mail messages sent from his private account.

“I miss you ridiculously,” began one of the e-mail messages from Roehm to Womack.

On another Womack had written: “Thanks for taking care of me.”

To which Roehm had replied: “We take care of each other . . . it’s the only way through this.”

Soon Roehm had secured Womack a permanent job at Wal-Mart.

According to Wal-Mart’s suit, she was “eager to have her new ‘friend’ working under her on a daily basis”.

As they hunted for evidence, Wal-Mart investigators persuaded big advertising firms to hand over confidential e-mail messages, dinner receipts and notes from meetings. Wal-Mart claims Roehm accepted a case of Effen vodka, valued at nearly $400 (£200), from the chief executive of Draft FCB, an ad firm. The gift violated Wal-Mart’s policies, but Roehm described it as “a HUGE hit” in a thank-you e-mail.

Wal-Mart investigators even tracked down the pair to a Holiday Inn hotel room in Guatemala where, with ear pressed to the door, the agent heard “moans and sighs”.

Roehm and Womack have denied they engaged in a sexual relationship or did anything wrong.

Last week, in a statement put out by her lawyer, Roehm accused the firm of running a “smear campaign” based on “anonymous witnesses and . . . selective use of e-mail, taken way out of context”.

Roehm portrayed herself as an agent of change who seemed to threaten some Wal-Mart executives. “Perhaps some did not like following or taking the advice of a woman,” she said.

The case continues, and Wal-Mart’s dogged investigators continue with their work. This is not the first time this year that Wal-Mart’s cops have pounced. Last month, the company fired a computer technician, Bruce Gabbard, and one of his superiors, Jason Hamilton, after uncovering evidence that Gabbard had taped phone conversations between members of Wal-Mart’s media-rela-tions staff and a New York Times reporter.

The team’s biggest scalp was Thomas Coughlin, once a protégé of Wal-Mart founder Sam Walton. In 2005, Coughlin was sacked amid accusations that he had used company funds to pay for items including beer, duck-hunting boots, a jeep and a dog kennel. His total theft, Wal-Mart said, amounted to more than $500,000.

However, there may be a price for all this success in prosecuting its own staff, said James Cox, law professor at Duke University. “Some of these things are better off being put quietly to bed — if you’ll excuse the pun. This kind of publicity does nobody any good — even if you are right.”

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