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There's a difference between evidence (objective or otherwise) and proof. While several auditees might tell the same lie, it's also possible that documents have been falsified in concert with the verbal prevarications. Whether there's evidence for something or a lack thereof, an eternal auditor has to work with what he's given or what he can discover in a limited amount of time and understand that there's very little absolute proof in any case.
Still it is a fact that the auditors have to do a lot of job in the given time that prohibit them from going into minute details.
Thanks for the response.

but here's my take: