A rash of car break-ins was reported in the Promontory area last Thursday night, but only about half were reported to police.
Most of the vehicles had also been left unlocked, police said.
An RCMP officer investigating calls from residents on Brewster Place, MacSween Drive and Rexford Drive found 20 vehicles had been entered, but only 10 were reported to police.
Police believe one person is responsible for all the thefts.
"Police are asking for people to lock their vehicles, and to avoid leaving valuables in plain view, even if your car is locked," RCMP Cpl. Tammy Hollingsworth said in a news release.
She warned car owners to never leave personal identification, vehicle ownership documents, cash or credit cards in in their vehicle.
Anyone with information about the Promontory car break-ins - or any others - to call the Chilliwack RCMP at 604-792-4611 or Crime Stoppers at 1-888-222-8477.