Drivers in Leicestershire should stay with their cars while de-icing windscreens to avoid opportunist thieves stealing cars, police said.
The warning comes as cold weather sets in and motorists leave their cars unattended with the engine running to help defrost windows.
Last year, 13 people had their cars stolen after popping into their houses while their cars warmed up.
Sgt Gavin Drummond from Leicestershire Police said thieves worked quickly.
"Modern cars are difficult to steal without their keys, so potential car thieves welcome the opportunity of taking vehicles which are left empty with their engines running," said Sgt Drummond.
"They pay attention to the forecast, get up early and go down residential roads which are often in otherwise low-crime areas. In seconds they can make off with a high-value car."
He added it was also an offence to leave an unattended vehicle with its engine running on a public road.