The numbers—specifically the Hobbs and the Tach—were not adding up.
The school used dispatch software that kept track of the hours flown so that oil changes and required inspections were not overflown. There was a 0.8 hour discrepancy in the Hobbs number of one of the planes, then a week or so later another aircraft had a 1.2 hour discrepancy. The discrepancies were found first thing in the morning by student pilots during their preflight inspections, leading the flight school to deduce the flights had happened after hours.
