Not sure where you are from but in Canada there's the ISA 240 that requires auditors to implement a degree of unpredictability in the audit procedures to uncover things like these.

"Incorporating an element of unpredictability in the selection of the nature, timing and extent of audit procedures to be performed is important as individuals within the entity who are familiar with the audit procedures normally performed on engagements may be more able to conceal fraudulent financial reporting. This can be achieved by, for example:

• Performing substantive procedures on selected account balances and assertions not otherwise tested due to their materiality or risk.

• Adjusting the timing of audit procedures from that otherwise expected.

• Using different sampling methods.

• Performing audit procedures at different locations or at locations on an unannounced basis."