Thieves drove off today with three cars left by owners to warm up on drives.
The cars all had their engines running while their owners were inside houses preparing to leave, said officers.
Police said they thought the thefts in Chelmsford and Ingatestone, Essex, were planned and linked.
"This isn't new, it's happened before in cold weather," said a spokesman.
"Thieves notice that people turn their engines on to warm their cars up and then go inside for a cup of tea, or whatever.
"And then they simply get in the car and drive away."
Police said the three thefts took place between 7.20am and 7.50am today.
A Ford Galaxy was stolen in Chelmsford and a Peugeot 407 and a Toyota Avensis were taken in Ingatestone.