A SAR/STR should be filed when the accountable institution identifies that:

According to the ACAMS CAMS Certification Study Guide (6th edition), one of the red flags for money laundering is the structuring of cash transactions to avoid reporting thresholds. Structuring is the practice of breaking down large amounts of cash into smaller deposits or withdrawals that are below the reporting threshold of $10,000 in the United States or equivalent amounts in other jurisdictions. Structuring is done to evadethe detection and reporting of cash transactions by financial institutions to the authorities. Therefore, when an accountable institution identifies that a customer is engaging in structuring or other forms of cash transaction manipulation, it should file a SAR/STR to report the suspicious activity1

 1: ACAMS CAMS Certification Study Guide (6th edition), page 64.