Goldmann also said that automated systems can't distinguish between legitimate and false receipts.

So just what are employees getting away with on their expense accounts? Upgrading hotel rooms, excessive taxi rides, double-billing for plane tickets, seats and baggage fees, luxury rental cars, fake lunches and dinners, layovers for personal visits and heaps of laundry on overnight trips, according to Goldmann, Geron and Bachman.

Whether these false charges amount to a bit of personal pampering or thousands of dollars of additional income, expense account fraud is a crime. It's just a matter of getting caught. Depending on company policy, fraudsters may be subject to immediate termination if the company finds out, Bachman said.

Goldman knows of a sales manager who lost her job when she abused the gift limit on her expense account. While 75% of the purchases were for actual clients, she also used the card to purchase expensive gifts under family names. Then, she sold those gifts on eBay and reimbursed herself.

It proves that while oversight and tight policy can reign in expense account abuse, employees are also getting creative.

"There are a million different ways to game the system," Goldmann said. But the fallout could ground high-flying business travelers.