I thought it was his voice in her flashback when she was calling for someone to come back for her. Her familiarity with his work with the Millennium Falcon suggested their professional relationship was more than just the transactions we witness. The way someone/thing scolded her when she dawdled cleaning the parts, that he paid her in rations, rather than a more commonly monetized currency, it all seemed to indicate that she worked for him in some capacity, though obviously not a slave.