On Amazon RDS, what consumes storage space beyond the database files?
AWS support came back with a comprehensive answer.  In this case, it was transaction log backups, being written to the SQL disk before offloading to S3.  There are a few more things which can consume space, mostly temporarily.  As their tech outlined:


  As a best Practice, I would recommend to keep at around 5% free space for similar processes (Log Backups, Error Log Files, Trace Files, etc), however this may vary based on high transactional volume or bulk data upload process , which may need high disk storage temporarily.