That's according to Pleasanton Crime Prevention Officer Archie Chu, who demonstrated recently how quickly a thief can get access to a locked car if there's something that could be valuable inside.
Chu said about a third of the thefts that occur in auto break-ins in Pleasanton are from cars that are left unlocked or with their windows open.
"When you make it that easy, you make it too easy for the bad guys," Chu said.
It's a simple fix to stop those crimes: Take a few moments and put valuables somewhere they can't be seen.
"It's about limiting opportunities to commit crimes," Chu said.
Hiding things under a jacket or bag doesn't work, he added.
A recent tour of a Pleasanton parking lot with Chu showed no obvious items in sight, but about a half dozen cars had bags, jackets and other items used to keep valuables out of view.
"One of the mistakes people make is hiding things under a jacket. That doesn't fool anyone," Chu said. He pointed out one case where a bag was used to hide electronics that had been left plugged in to charge.
Crime statistics from the Pleasanton Police Department show 53 auto burglaries through the end of March, while at the same time last year, there had been 56, a drop of about 5%.
There were 100 thefts from unlocked vehicles through March of this year, while there had been 80 thefts from unlocked vehicles as of the end of March 2012, a rise of about 25%. Chu described the majority of the thefts from unlocked vehicles as crimes of opportunity.
Most were in residential neighborhoods, but a significant number were from cars at Stoneridge Shopping Center or parked along commercial streets. Chu said to take extra precautions when headed out to shop.
At the mall or downtown, he said, "Move things to the trunk before you get to your destination."
The Pleasanton Police Department is starting a new campaign to help stop auto burglaries and other property crimes. The program is similar to the "If I Were A Thief" program in Danville. There, volunteers leave a card if they find an opportunity for a crime.
In Pleasanton, volunteers and patrol officers will leave a tri-fold pamphlet on a car window or in a door jamb.
The pamphlet will contain the time police or volunteers were there, with items checked off to let a resident know the opportunity at the site for a crime, such as property left inside a vehicle, doors or windows or a garage doors left unlocked or open, or newspapers or mail accumulating at a home.
The tri-fold was specifically designed not to look like a citation, and states that it is a courtesy notice from the Police Department.