A simple wing mirror mistake could be a tell-tale sign to thieves that a cars can easily be broken into and stolen, it has been warned.
Vehicles with unfolded electric wing mirrors could indicate it has been left unlocked, the AA and police said.
Convicted car thieves spoke to researchers in detail about how they look out for expensive cars with wing mirrors sticking out.
Cars that are left unlocked are an easy target for opportunistic thieves and can lead them to steal any valuables left inside.
Former police chief inspector Dr Kevin Floyd, a criminologist at Huddersfield University, interviewed convicted vehicle thieves in prison.
He saud that many of the thefts came after "lazy" motorists failed to lock their cars even when they had valuables stashed inside.
Dr Floyd said: "With modern cars nowadays, open wing mirrors equate in the thief's mind with an open door. It's as simple as that. It's a green flag. It's as easy as walking along the street.
"They don't want to try the doors because they are so sensitive they will set off an alarm but if the mirrors are open, then it's confirmation the car is unlocked and no alarm will go off.
"They can then sit there, pretend to be the owner and take as long as they want to have a good look around and either nick things or steal the car. It's an open invitation."
The criminologist added: "The other basic mistake is leaving valuables in the first place because the car owners are lazy.
"It is not just valuables with a financial value but valuables that can be used to commit more theft. For example, people may leave a bunch of house keys and a letter with their address in there."
Jenny Sims, the National Police Chiefs' Council's expert on car crime, echoed the criminologist's comments after the Office for National Statistics (ONS) revealed that car theft surged by 22% from around 90,000 cases to nearly 110,000 in the past year - equivalent to 300 a day.
She said: "Locking your vehicle, even when filling up or parked on your drive, greatly reduces the possibility of it being targeted by an opportunist thief.
"Even if you have locked your vehicle, check you haven't left any windows or the sunroof open.
"Vehicles today are by and large more difficult to steal than ever unless the thief can access your key or fob to clone them."