Officers from Wiltshire Police visited two car washes in north Swindon on Tuesday August 20 as part of a strategy to tackle modern slavery.
Sergeant Nathan Perry, who helped plan the operation, said the cash-in-hand businesses could be used as fronts for organised crime gangs.
The police officers were joined on the operation by two immigration enforcement officers and spoke to workers and checked papers.
The owner of one of the businesses, 5 Star Hand Car Wash on Cricklade Road, was issued with a civil notice after officers found a Kurdish man suspected of working illegally at the firm. If the employer is found guilty the maximum fine is £20,000.
Police said the visits were part of a bid to build intelligence and improve relationships with car wash business owners.