A field audit
In the 1983 film Superman III, a company discovers that money is being cleverly stolen, and despairs of finding a thief that cunning. Just then they hear a squeal of tires, look out the window, and see Richard Pryor, a junior programmer, arriving in his new, expensive, red, expensive, imported, expensive, sports car. Management considers this to be A Clue.
Companies need to be actively looking for clues, and acting on them when they appear. There are a lot of ways that this is done, from checking regularly to see if there are phantom employees (they tend to get a salary but no benefits), through following procedures know to reduce the opportunity for theft, to watching for anomalous behavior.
As an example, someone mentioned in passing to a company owner that one of his employees in a branch office repair department was spending a lot of time with prostitutes. Since this employee didn’t make enough money to afford to do this, the owner recognized this as A Clue.
He sent in a crew on a Sunday and they went through the books and records, all of which were fine. They counted the inventory, which matched up. Then they actually started opening all the boxes and looking at the inventory.
What they discovered was that the employee had been selling the new parts, and replacing them with old parts.
A significant number of companies go out of business because of employee theft, and any audit that depends entirely on cooked books with no verification represents a failure to exercise due diligence. It behooves you to be aware of this problem, make an effort to hire screened employees, and work to minimize the effects of dishonest employees who slip through your background checks.