Police chiefs in Sheffield are urging residents to be on the look-out for slaves being forced to work in cannabis farms.
With a new police team set up to tackle modern day slavery, police chiefs want people to report their suspicions.
The force said modern day slaves are often forced to take part in illegal activities against their will, including cultivating cannabis, begging and pickpocketing..
Superintendent Bob Chapman said: "Is there a property on your street were the curtains and blinds are closed all the time? Or is there a pungent smell coming from the property? Maybe there are people visiting at unusual times of day or night.
“These are all signs that can be reported to us by calling 101, or 999 in an emergency – we need these pieces of information, which some people may see as being insignificant, to help us tackle modern slavery in South Yorkshire.
“Victims will be treated as just that – victims – even if they are being forced to take part in illegal work against their will.
"It's hugely important that we continue to encourage people to report modern slavery and the signs of it to us. Whether you think it is happening to you, or to someone you know, please contact the police as soon as possible."
Call South Yorkshire Police on 101.