Expensives can be hidden at the corporate level but used personally. The company plane can be used for both personal/business and those are hard to separate.
There are actually very detailed rules about how personal use of corporate planes needs to be reported for both tax purposes and shareholder reporting purposes.
Even if a plane is already going from City A to City B for business purposes, if the CEO brings a spouse along for personal reasons, there can be reporting consequences. Of course, this only matters if the company is under sufficient scrutiny to actually report such details.
