Car hackers are able to steal a $113,000 BMW simply by holding a bag up

Doing little more than holding a bag up to the front door of a house, this is a thief stealing a BMW.

Days after a Mercedes was said to have been taken in a similar way, the £60,000 ($113,183 NZD) BMW X5 vanished from its owners' drive as they slept.

The thieves were caught on CCTV using some sort of transmitting device inside the bag which is thought to extend the signal from the car's keyless fob which was inside the house, reports Daily Mail.

The car uses a keyless start system meaning it can be unlocked simply by having the fob close by. Both vehicles were stolen in Essex - prompting fears a gang is targeting new "keyless" cars with high resale values.

Security expert Ray Anderson, whose firm covers Essex, said it was the fourth such theft he had heard of in the last four months.

He warned the only way to protect against it may be to keep key fobs inside a metal box. There has also been advice to keep them in the fridge.

"The metal blocks the signal," said Mr Anderson, of Classic Security Solutions. "We think these keyless fobs continually emit a signal. You can turn them off but most people don't.

"We think, from analysing CCTV, [the thieves] are using a device to extend the signal which makes it appear the fob is closer than it is.

"Nothing special is required to use it. This whole thing was over in five minutes and there is not much chance of them getting it back."

The owners were asleep when the car was stolen from their driveway at around 2am on April 4.

They only realised it had gone the next morning. They did not want to be identified but have released the CCTV footage from their security cameras to warn others.

They said: "We are extremely concerned our BMW could be stolen in this way. We see this as a significant security breach."

In the CCTV, two men can be seen sprinting towards the house. One goes to the car door while the other, holding a bag, stands by the front door of the house.

One of the men can then be seen moving the bag around outside the front door, apparently trying to detect a signal.

Moments later, the car lights come on and the two men are inside and away. CCTV also showed the £35,000 ($66,023 NZD) Mercedes C220 being taken from a driveway in Grays on April 11.

In this case the thieves went through a similar routine, with one man waving a bag near the wall of the house.

Owners Paige Foster and Richard Haydon said the fob was stored at the back of their home, far from the drive.

But they believe the thieves managed to hack it and extend its reach. Keyless vehicles do not have traditional ignition keys, instead coming with a plastic fob that contains a computer chip and security code.

When the fob is nearby, the code is detected by the car's computer, allowing the driver to start the engine at the press of a button.

It is thought that thieves could be using a "relay" strategy with a pair of radio transmitters. One gets as close as possible to the key fob inside the house by holding a transmitter close to the wall.

The second holds his device by the car door. The devices relay a signal to unlock the vehicle.

Mercedes said it was not aware of any thefts relating to keyless start systems and said all its vehicles had extensive security systems.

A BMW spokesman said it was hard to tell from looking at stills of the theft how the car was stolen. He added: "This form of theft would appear to be extremely rare."

An Essex police spokesman said: "We take car theft seriously and use a range of tactics to detect this."