Well, for one, they did see $20m were stolen, since they've brought up charges against this individual for stealing it.
$20m over 4 years is $5m/year. That may sound like a lot of money, but for a company that has $500m/year of operating revenue, isn't really that much, and when you are in charge of accounting for that money and know how to work the books, it isn't that hard to do.