As the old logical fallacy goes -[ you can't prove a negative](http://rationalwiki.org/wiki/Negative_proof), and[ absence of evidence](https://en.wikipedia.org/wiki/Argument_from_ignorance#Absence_of_evidence) is not[ evidence of absence](https://en.wikipedia.org/wiki/Evidence_of_absence). While for a long time this might've been true in various financial audits - you could only prove that some invoice existed, not that there were no invoices you missed.