You might think that the front line for preventing human trafficking would be at airports and international borders, or in police raids.
More likely, it would be at the hotel front desk.
The international hotel chain Marriott has completed a two-year project to train half a million staff with the aim of spotting potential victims of trafficking, according to the BBC.
Marriott's workforce, in almost 7,000 hotels, have completed a process of mandatory training teaching them to look out for warning signs.
"Hotels can unfortunately be unwilling venues for this unconscionable crime," David Rodriguez, the hotel group's chief global human resources officer, said.
"There is no easy fix but combating modern-day slavery starts with awareness," Rodriguez said.
"And we now have a significant number of people capable of recognising suspicious behaviour and reporting it to management and, in some cases, law enforcement."
The type of signs might include "guests with minimal luggage and clothing" and "individuals who can't speak freely or seem disoriented".
There might be "guests who insist on little or no housekeeping." Concerns about trafficking for prostitution might be raised by multiple people being escorted to a room one at a time.
Staff are taught that while none of these individually might be a sign of trafficking, when there is a "combination of indicators", it might be time to raise concerns with hotel managers.
A spokeswoman for Marriott said the training had recently yielded result when staff in a central London branch had noticed someone "using the hotel lobby to meet with and groom an under-age girl".
Police were contacted and the perpetrator was subsequently jailed.
Last month, researchers from George Washington University published a paper showing how artificial intelligence could be used to identify hotels in the US being used for trafficking.