How do I reconcile discrepancies in small fleet fuel card records?

When I first took over the weekly fuel card reconciliation for our local service fleet, I encountered an easily underestimated issue: a fuel card showed two refueling transactions on a day when the driver was on leave. The amounts weren't large, but they didn't match the shift schedule. It wasn't the kind of glitch where the cause is obvious at a glance; it was more a combination of several…