Two Mercedes cars worth a total of £34,000 were stolen from the driveway of a house in Surrey after thieves broke in and took the keys.
Police said the keys to both vehicles were in a dish by the front door of the house off Horsham Road, in Cranleigh.
The owner discovered the cars had been driven away at 0530 GMT on Tuesday.
"Don't advertise your car keys to thieves by leaving them on display," said Det Con Dan Chant, of Surrey Police.
"It may seem extreme but taking the car keys to bed may be an easy way to protect your vehicle.
"Don't use the obvious places such as pots and tins by the door or in the hallway as these are the first places burglars will look."