Synchronization bits is what I was wondering about: https://docs.aws.amazon.com/AmazonS3/latest/userguide/s3-fil...

> For example, suppose you edit /mnt/s3files/report.csv through the file system. Before S3 Files synchronizes your changes back to the S3 bucket, another application uploads a new version of report.csv directly to the S3 bucket. When S3 Files detects the conflict, it moves your version of report.csv to the lost and found directory and replaces it with the version from the S3 bucket.

> The lost and found directory is located in your file system's root directory under the name .s3files-lost+found-file-system-id.