Officers have launched a new drive to investigate possible offences across the district.
Detectives, immigration officials and members of the Environment Agency visited businesses employing manual labourers last week to speak to workers.
Chief Inspector Kate Riley of Wakefield District Police said: “Human trafficking and modern slavery are, quite simply, crimes which should not be taking place in the 21st century.
“Sadly, it most likely is the case that activity of this type is ongoing in the Wakefield district.
“This was a significant operation conducted with partners to visit businesses, interview staff and gain intelligence about what has been happening
“During the course of the operation officers spoke with persons who could possibly have been victims of trafficking.
“While those they spoke to did not request help or come forwards, they were given information about the services available to help them break free, if they were under any form of duress.
“The intelligence we gained is being investigated and more operations are planned for the coming months.”
Police said no one came forward to say that they had been trafficked or exploited but “a number of pieces of intelligence were received”.
The operation was the first of its kind in the district and was part of a series of planned activity in the district.
Officers have also been working with charity Hope for Justice, which aims to end human trafficking and modern-day slavery, as well as Wakefield Council and West Yorkshire Police’s Human Trafficking Team.
If you have been the victim of human trafficking or modern day slavery call police on 101. Information can also be passed to Crimestoppers anonymously on 0800 555 111 or to the modern slavery helpline on 08000 121 700.