A top-of-the-range Jaguar was among the cars found unlocked during an overnight operation by South Yorkshire Police.
Officers checked 614 vehicles parked in various locations across South Yorkshire.
Of those 614 vehicles, 11 were found to be unlocked, including a top-end Jaguar thought to be worth around £24,000.
Those insecure vehicles also included work vans and small family cars.
After finding an unlocked car, officers then woke the owners and advised them to secure their vehicle.
The owners were also given an advice leaflet on keeping their property safe.
In one case, two vehicles found parked on the same drive in Bessacarr were found to be unlocked. The occupants of the house were woken at 1am and advised to lock their vehicles.
Chief Inspector Iain Chorlton said: “People are making it far too easy for criminals to either steal their vehicles or steal items from them.
“For most people, their cars or vans are among the most valuable things they own, yet a surprising number of people are leaving them insecure and vulnerable.
“Check and double-check that vehicles are locked after parking them up and, if possible, also park in well-lit and safe areas to stop potential thieves in their tracks.”
Anyone with information on car crime in South Yorkshire should call police on the non-emergency number, 101, or Crimestoppers, anonymously, on 0800 555 111.