Following an increase in reports of items being stolen out of cars, officers patrolled the Jupiter Gate and Neptune Gate area of Stevenage over the weekend.
"Unfortunately, they found 12 cars which had been left unlocked," said a spokesperson for Hertfordshire police.
"Please always lock your vehicle when you are leaving it unattended, and remove all items from inside. Even if you are parking on your driveway, you must lock your car and check it has locked.
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"We know that many thieves are opportunist and will simply target roads and then try car doors as they wander down.
"Don't make their day - always lock it when you leave it!"
You can report information online at herts.police.uk/report or call the non-emergency number 101.
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