I’m not sure of your whole scenario, but generally an offload to cloud storage plugin will only show the images or files that are uploaded through it and the WordPress interface. Depending on how your s3 configuration is, generally files uploaded there are viewable through a verification process even if they are public viewable. In other words if someone that wasn’t logged in to your AWS console with access to your s3 bucket tried to access files on it without going through your website, their access would be denied and an error message would be shown. My point being, each file uploaded to an s3 bucket is authenticated as it’s uploaded by use of your s3 key files. So if a file wasn’t uploaded through your site, Offload Media would be denied access. If I misunderstood your question, then my answer is I don’t know. Hopefully this helps you or at least somebody.