Andy Mallon found a bug with Filestream:
I tracked the problem back to this specific scenario: When you have a database that is part of an Availability Group, and you drop a table that contains filestream data, the filestream garbage collection does not clean up the data container subdirectory that corresponds to that table. Garbage collection will continue to clean up other items (eg, deleted rows), but the dropped table never gets cleaned up.
I’ve logged this as a connect item for your up-voting pleasure.
Go forth and upvote.