The idea of crawling for S3 content has been around for a while (see this post from 2011 https://digi.ninja/blog/whats_in_amazons_buckets.php for one example).

Also google can end up indexing content in places like S3 or Azure file stores if there's a link there from another piece of content that it indexes. (for example https://community.rapid7.com/community/infosec/blog/2013/03/...)