You can do real-time monitoring of API calls by directing CloudTrail logs to CloudWatch Logs and establishing corresponding metric filters and alarms.
Security groups are a stateful packet filter that controls ingress and egress traffic in a VPC.
CIS recommends that you create a metric filter and alarm for changes to security groups. Monitoring these changes helps ensure that resources and services aren’t unintentionally exposed.
Security groups control the ingress and egress, changes could be made to maliciously allow egress of data or external ingress. Without alerting, this could go unnoticed.