Doncaster Council staff went out on secret snooping missions three times this year, according to an official report.
The report, submitted to the audit committee, showed that all three surveillance operations took place in April.
One of the investigations was in relation to underage alcohol sales, and the other two were over counterfeit goods.
Each investigation is on-going.
In one of the counterfeit sale cases test purchases were made.
No further information is available about any of the cases.
The council is allowed to conduct secret surveillance to prevent or detect crime under the Regulation and Investigatory Powers Act 2000.
Investigations must be approved by magistrates in circumstances where the suspected crime is punishable by at least six months’ imprisonment or involves selling tobacco and alcohol to children who are under age.
Previous investigations by the council in relation to underage sales and counterfeit goods have led to legal action.
However, the number of applications submitted to the magistrates for approval has greatly reduced in recent years.
In one October 2012 case, the perpetrators of trademark infringement were arrested and goods were seized.
One defendant was given a six month prison sentence, suspended for two years and community service.
Another received a fine and community service.