Modern vehicles have made several strides in anti-theft systems, making it more difficult for a thief to steal a vehicle after breaking into it. In an increasing number of cases, however, vehicle owners unwittingly make themselves vulnerable to theft by leaving their keys in a vehicle.

According to the National Insurance Crime Bureau, an organization dedicated to detecting and preventing insurance fraud and vehicle theft, there were 147,434 vehicle thefts in the United States between 2013 and 2015 where thieves used a vehicle's keys to steal it.

Between these years, thefts involving unattended keys increased by 31 percent. There were 57,096 thefts in 2015 involving thieves who gained access to a vehicle's keys, a 22 percent increase from the previous year's total of 46,695. In 2013, 43,643 vehicles were stolen in this way. The NICB says the actual numbers may be even higher, since vehicle owners may be reluctant to admit that they left the keys or FOB in an unlocked vehicle.

"Anti-theft technology has had a tremendous impact on reducing thefts over the past 25 years, but if you don't lock it up, it's not going to help," said Joe Wehrle, president and CEO of the NICB. "Complacency can lead to a huge financial loss and inconvenience for the vehicle owner. Leaving a vehicle unlocked or with the key or FOB inside gives a thief the opportunity to take not only the car, but also any possessions inside. It can also provide access to your personal information if the registration is left in the glove compartment."

The NICB says thieves who are able to access this personal information will be able to find out where an owner's residence is. In some cases, vehicle thieves have driven to this address and burglarize the home as well.

Vehicle thefts where the keys or FOB were accessible were most common in California, where 22,580 of these thefts took place between 2013 and 2015. Another 11,003 occurred in Texas, with 9,952 thefts taking place in Florida. Hawaii was the only state where no thefts were reported as a result of keys left in the vehicle.

In Connecticut, 1,471 of this type of vehicle theft were reported between 2013 and 2015. During the same period, 778 incidents of these incidents were reported in Rhode Island.

Vehicles stolen in this way were usually recovered. Only 6 percent of the reported thefts, or 8,759 vehicles, remained unrecovered by the time the NICB accessed the data. The recovery rate was 94 percent in Rhode Island and 94.5 percent in Connecticut.

The NICB recommends that drivers always lock the vehicle, set an alarm, and take their keys and FOBs with them when leaving the vehicle. Vehicle owners may also not want to keep their garage door opener or any documents with personal information inside the vehicle.

The majority of thefts where keys were left in the vehicle occurred during the fall and winter, likely as a result of drivers warming up the engine. Do not leave the vehicle unaccompanied after you have started it. You should also reconsider whether you need to let the vehicle idle at all; modern fuel injection systems allow you to drive off soon after starting your vehicle without damaging the engine, and doing so will warm up the interior faster than idling.

Never leave the car idling and unaccompanied, even if you are planning to make just a quick stop. Many vehicle thefts are crimes of opportunity, and a person could decide to make off with a vehicle if they see that it is empty and has the engine running.