Janitor Monkey detects AWS instances, EBS volumes, EBS volume snapshots, and auto-scaling groups. Each of these resource types has distinctive rules for marking unused resources. For example, an EBS volume is marked as a cleanup candidate if it has not been attached to any instance for 30 days. Janitor Monkey determines whether a resource should be a cleanup candidate by applying a set of rules on it. If any of the rules determines that the resource is a cleanup candidate, Janitor Monkey marks the resource and schedules a time to clean it up.

via Netflix Open-Sources ‘Janitor Monkey’ AWS Cleanup Tool.