Owners of keyless-entry cars are being warned to be vigilant after two theft incidents in Nottinghamshire.
It is believed a device may have been used to replicate or extend the signal from the keys in the house.
Both of the vehicles were BMWs and were taken from the Mapperley area in the early hours of Tuesday, April 30, without the keys being taken and without any signs of forced entry.
One was a BMW 4 Series taken from Dawlish Court at around 3.45am. The vehicle was later recovered by Lincolnshire officers in Lincoln Road, Sleaford, but the occupants fled the vehicle.
The other was a BMW 320i XDrive Sport, taken from Bradstone Drive at around 4.05am. It was later spotted on the A614 in Arnold and failed to stop, before being abandoned in Crookdole Lane, Calverton.
Both vehicles have been recovered by police but no arrests have been made at this stage. Anyone who saw anything suspicious is urged to call 101.
Inspector Chris Pearson, Neighbourhood Policing Inspector for Gedling, said: "Nationally thieves are targeting vehicles which have keyless-entry systems using digital scanners allowing the car to be unlocked without a trace of forced entry or damage.
"Regular remote locking fobs which require the car owner to press a button are not vulnerable to these crimes, it is only those cars which allow the owner to approach and unlock the car with the keys still in their pocket.
"I would urge anyone with a keyless entry system to check whether the fob can be switched off overnight. Speak to your dealer about software updates and whether new key fobs with added security are available.
"People should store keys away from household entry points. A keyless fob should be stored as far into your home as is possible, hampering a criminal’s ability to detect and relay its signal.
"Signal blocking pouches (Faraday pouches) will block the signal from a keyless entry fob. Keeping your keyless entry fob out of sight is not enough – thieves only need to gain proximity to the key to amplify the signal."