The warning comes after a spate of thefts from cars throughout the city, with incidents in Douglas, Blackrock and Mayfield all reported recently.
Superintendent John Quilter said that thefts from cars are on the increase and one of the biggest contributors is people leaving them unlocked at night.
He reiterated the need for the public to take precautions to protect their valuables.
“We have had cars not broken into but entered because they were unlocked and we have had cars taken because they were not locked as well,” Supt Quilter said.
“A huge percentage of the cars that are being entered or taken have arisen as a result of cars being left open and in some cases the keys being left in them. It is frightening.”
Supt Quilter said the figures showed that well over 50% of reported thefts from cars were cases of them being left open.
“It is a city-wide problem,” he added, noting a surge in the Douglas area in particular. The Superintendent also said that the force has had some success in relation to the issue.
“There is a gang operating and we have targeted them and had some success, but that is not to say people should become complacent,” he added.