There have been 62 thefts from cars in the St Albans district since the beginning of the month.
Silver pocket watches, briefcases, and laptops have all been stolen from unsecured cars.
Acting Det Insp John Harlock said: “These types of crimes are often opportunistic and thieves will try car doors to see if they are open, and move on if they find it locked.
“However, if you leave something valuable on view they may take the trouble to break into the car, meaning you not only lose the item but may have a hefty repair bill as well.”
Between March 1 and March 23, there were 29 offences in St Albans, nine in Harpenden, and 24 in London Colney.
Det Insp Harlock said: “I would appeal to anyone who witnesses any suspicious activity around vehicles to contact police straight away.”
“If a crime is in progress, always dial 999. Otherwise, please call the non-emergency number 101.”
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