Lost? Navigation devices, many with your choice of sexy voices in any language, can tell you where to turn.

When those portable global positioning system units are stolen, however, they can tell a crook the fastest getaway route - but not tell the rightful owner of the unit where the thief is heading.

Thieves are swiping GPS units at such speed across country that many police departments are warning owners not to leave them in parked vehicles.

So far this year, 810 GPS units have been stolen in Philadelphia, with Center City and the Northeast hit the hardest. They now account for 20 percent of all thefts from vehicles, said Lt. Frank Vanore of the Public Affairs Unit.

City police advise motorists to always remember to pick up the telltale suction cup that attaches the GPS to the windshield and to wipe away the identifiable ring left on the glass.

"Take it [the cup] out of the car and take a minute to clean the windshield," Vanore said. "It's like a flag for the bad guys that the car has one."

Philadelphia police are even tucking warning notices - "Don't become a victim!" - on windshields.

Lt. William Kushina of the Cherry Hill Police Department said that about 50 such thefts had been reported there in the last month, mostly GPS units and satellite radios.

"We're getting hit in our shopping centers, and we're getting hit in our developments, too," Kushina said. "Lots of [thieves] are going into unlocked cars."

Catherine Rossi, spokeswoman for the Mid-Atlantic office of the Automobile Association of America, said some police departments were reporting that thefts from vehicles had doubled, mainly because of stolen GPS devices. Most car insurance policies don't cover the loss, Rossi said.

Sam Kimelman, 19, learned that the hard way in December when he returned to Philadelphia from college. His unit was stolen from his Subaru Outback parked in Center City.

"I went out to dinner and was going back to the house and saw that the window was smashed in," Kimelman said.

It cost him about $500: Insurance didn't cover the GPS loss, and he had to pay to replace the window.

In Boston, the number of thefts jumped so dramatically - from fewer than 300 in 2006 to more than 1,000 in 2007 - that it has skewed crime statistics, officials have said.

In Upper Merion, Lt. James Early said, 127 GPS units and 64 laptops were stolen last year, most from vehicles where the items were easily seen - and many in the parking lot of the King of Prussia mall. Some surveillance cameras captured shots of thieves using a punch to break windows.

"They just reached in and took the GPS. It took all of about five seconds to do the theft," Early said. "When you leave something visible from the outside, you're just inviting thievery."

The devices, most no larger than your hand, sell on the black market for $100. New, the units range from a couple of hundred dollars to more than $1,000.

"Whatever you paid for it, they're going to sell it for pennies on the dollar. They don't care; they just need their next fix," said Capt. Jack McGinnis of the Northeast Detective Division. "They're usually selling them for drug money."

Some companies are now producing software for theft protection that can track the whereabouts of units, and some manufacturers are requiring that codes be entered before the system can be activated.

"If someone steals your GPS device, they've stolen more than just your device," said Ken Westin, chief executive officer of GadgetTrak, in Portland, Ore. "It has your home address and where you've been for the past week."

Philadelphia Police Officer Raymond Johnson said he had a GPS and attached it to the center console instead of the windshield. He said he never left it in his car unattended.

"You have to be innovative," Johnson said. "Keep one step ahead of the thief. Where will this device still work but be inconspicuous?"

Contact staff writer Barbara Boyer at 215-854-2641 or bboyer@phillynews.com.