Doncaster Council bosses have used special powers to snoop in the borough eight times in the last 14 months.
Officials were given the go ahead by magistrates to spy on people or shops as part of investigations into illegal sales of tobacco or alcohol.
The figures were revealed in a report to Doncaster Council's audit committee.
The secret, directed, surveillance led to two warnings and three prosecutions, legal bosses at the authority said. Those were through operations carried out in July and in January. But one case, an investigation into illegal tobacco sales in five locations, ended with the complaints not substantiated.
Three of the schemes listed were part of currently ongoing operations.
Coun Iris Beech pointed out only one of the investigations was into illegal sales of alcohol.
She said most of the cases had been into illegal tobacco. In the investigation which involved sales of alcohol, the shop passed the test purchases which were carried out on the second occasion that they were performed.
"Tobacco sales are one thing," she said. "The one we have had where alcohol has been sold is particularly worrying," she said.
Councillors also asked why one of the investigations was for no specific location.
Helen Potts, principal legal officer at the council, said: "This was an investigation into an into an individual working with a number of different off-licences and at their own home address. The individual was thought to be active in that area so we focused on him, not any specific premises."
Councils are only allowed to spy on people where they are trying to stop a criminal office which would be punishable by more than six months in jail, or to investigate the sale of tobacco or alcohol to children. They must be approved by a magistrate before they take effect.
The committee noted the report.