The 'AWS EC2 Deprecated AMI Discovery' rule aims to identify when users query for deprecated Amazon Machine Images (AMIs) within AWS. Such queries may suggest potential reconnaissance by attackers looking for vulnerable systems, as deprecated AMIs could be more susceptible to exploitation. This rule uses AWS CloudTrail logs to detect 'DescribeImages' API calls with a parameter for deprecated AMIs. The analysis includes various investigative steps such as identifying the user, examining the source of the request, and validating the context of queries. While handling false positives from legitimate testing or development activities, it emphasizes the importance of investigating AMI usage security risks and suggests actions to mitigate vulnerabilities from potentially insecure AMIs.