Access Analyzer for S3 monitors your access policies to ensure that the policies provide only the intended access to your S3 resources. By default, all buckets and objects created in S3 are private by default. The new feature is available at no additional cost in the S3 Management Console in all commercial AWS Regions, excluding the AWS China (Beijing) Region and the Amazon China (Ningxia) Region. Once enabled, the feature will alert you to buckets that are shared outside of your AWS account and at risk of unintended access.”]
Source: https://aws.amazon.com/blogs/storage/protect-amazon-s3-buckets-using-access-analyzer-for-s3/