All sexual-related offences are under-reported. I think it just falls under that umbrella. Of course, people who are marginalized are less likely to feel confident to approach the police for help and are sometimes more likely to shirk away from the police.
My clients are complainants in a criminal matter and also are in conflict with the law themselves. That can make it very difficult for victims of human trafficking to engage with the police and report. I do think it also falls under that greater umbrella of under-reporting those sexual assaults.