Three control problems in one vehicle
Van sales carries inventory, cash and a selling route in a single unit that is out of sight for most of the working day. Each of those needs a control, and the failures interact: a stock variance can be a recording error, a genuine loss, a scheme issued without documentation or a return that was never entered. Which is why reconciliation has to happen daily and at the level of individual transactions rather than as a monthly summary.
The good news is that the daily cycle is short and repetitive, so a well designed process becomes routine quickly. The bad news is that any gap in it will be found and used, not necessarily dishonestly, but because people work around friction.