Gangs are raiding cars at motorway services using radio jamming devices to stop owners locking them.
Police are probing a wave of thefts where there were no signs of a break-in.
They believe crooks are lying in wait using £50 gadgets bought online to block remote key fobs sending locking codes.
Then they pounce while the owner is away.
Tony Forrester, 58, had a wallet and laptop lifted at an M40 Welcome Break near Oxford while buying a lottery ticket.
“It’s only when you get home that you realise and the thieves are long gone,” said Tony, of Walsall.
“I went back to the services and asked to check their CCTV, and they told me they didn’t have any in the car park. I’ve emailed Welcome Break, saying you’re putting customers’ safety and security at risk by not having CCTV.”
There was no CCTV in the car park. In summer there were 28 jammer break-ins at Cherwell Valley Services near Bicester 18 miles away.
Gangs raided 14 vehicles at three services on the M4 in just two weeks in November 2016.
A Thames Vally police spokesman said at the time: “Often victims are locking their belongings in the boot of the car and leaving the vehicle unattended for no more than five minutes.
"In that time, the vehicle is being entered and items are being stolen. We have cause to believe that jamming devices are being used.
”Our advice for motorists stopping at motorway service stations is to look and listen for confirmation that their vehicle has locked before walking away from it.
"If they are not able to lock their vehicle using a key remote, this could be a sign that a jammer is being used nearby.”
RAC spokesman Simon Williams said: “For years car makers have successfully used technology to make it harder for thieves, but sadly they have now got their hands on their own tech.
"The advice to drivers using any busy car park where thieves may be lurking to jam signals from their key fobs is to physically make sure the car has locked before they walk away.”
