Empty S3 bucket has objects that aren't multipart uploads

This is an unusual situation where your bucket appears empty but metrics show significant storage usage. Since you've already confirmed there are no multipart uploads or object versions, here are some approaches to investigate and resolve this issue:

1. Try listing objects with a different approach:

• Use the AWS CLI command aws s3 ls s3://your-bucket-name --recursive to list all objects
• Try the aws s3api list-objects-v2 --bucket your-bucket-name command which might reveal objects not visible in the console

2. Check for objects with special characters or unusual prefixes:

• Objects with special characters in their names might not display properly
• Objects with prefixes that look like directories (ending with "/") might be hidden

3. Look for delete markers:

• Even though you mentioned versioning was never enabled, it's worth checking for delete markers with aws s3api list-object-versions --bucket your-bucket-name

4. Check S3 Inventory:

• Set up S3 Inventory for your bucket, which provides a scheduled list of your objects and their metadata
• This might reveal objects that aren't showing up through other listing methods

5. Contact AWS Support:

• If none of these approaches work, AWS Support can help investigate the discrepancy between your bucket's apparent emptiness and the reported storage metrics

6. As a last resort:

• Create a new bucket and migrate any needed data
• Delete the problematic bucket entirely

The discrepancy between metrics and visible objects suggests there might be hidden objects or a potential issue with how the bucket's contents are being reported.
Sources
Emptying a general purpose bucket - Amazon Simple Storage Service
Amazon S3 examples using SDK for Java 2.x - AWS SDK Code Examples

The bucket metrics that S3 reports via CloudWatch metrics aren't real-time. They are updated approximately once per day. Also, you cannot delete an S3 bucket as long as it contains any objects or incomplete multipart uploads. The bucket must be completely empty in order to be deleted.

You made your checks exactly correctly. You used the ListObjectVersions API to check there are no objects committed to the bucket and the ListMultipartUploads API to confirm there weren't any incomplete multipart uploads in the "staging area" of the bucket not yet committed to the bucket as complete objects. When both those API calls return empty results, your bucket is definitely empty, regardless of what the metrics updated on a best-effort basis once per day are showing.