Those telltale suction cup marks on your windshield can make your car a target for thieves.

Diane Macdonald/Photographer's Choice/Getty Images

If you want to prevent your car from being a target for thieves, one of the first things a car-theft expert will advise you to do is to clear out any trace of gadgets and accessories that might be resold easily. You might put GPS units and iPods in the trunk, hide the power cords and sweep the interior to make sure there's nothing left out on display. And yet one of the most important things is often the most easily overlooked: the suction-cup marks on your window or dashboard.

Most GPS and car break-in thieves are looking for something they can snatch quickly and sell, including cheaper items. In fact, most car break-ins are perpetrated by the homeless and those addicted to drugs: They don't care about high-value items or even getting the best price, they just want enough money to get by. It's worth smashing in a window just to check under the seats and try the locks on your console and glove box.

While you may have paid hundreds of dollars for your GPS, they're often resold for less than $100. In fact, GPS thefts have doubled in recent years, and one county in Maryland reported a whopping 328 percent jump in just one year, from 2006 (189 reports) to 2007 (620 reports) [source: Minton]. Consider also the price of repairs (for that smashed window or ripped-up locks), the time you have to spend with insurance and law enforcement, and a rental car while you wait for repairs, and it adds up.

If thieves are walking down your street or through your parking garage, they're looking for a car that's both easy to break into and worth the trouble. Those little suction cup marks make your car a target. They're easy to see in the daytime and even easier to see at night, when many potential thieves are checking every car on the street with a flashlight. What they tell a potential thief is that there's probably a GPS device located somewhere in the car, and you've likely stowed it in the glove box or console.

But that's not the only danger those little marks might get you into. Next, we'll see how losing your GPS to a thief can lead to even more trouble.