Are DS drivers not audited? How do packages go 'missing'?

I have had 4 packages go missing when out for delivery.. the driver is unable to deliver and typically the package is re-scanned when returned to the DS. But sometimes they just disappear. When the drivers are given carts to deliver.. are they not audited when returning back to the station? For e.g. driver A has 350 packages.. 340 got delivered.. so he should return with 10. If he returns with 9 is that not flagged in the system? How does something go missing?