Caution: thieves working

GPS units, the portable digital devices that provide directions for drivers, are proving to be as popular with thieves as they are with car owners.

Police in Braintree, Brockton, Canton, Norwood, Foxborough, Mansfield, Rockland, and Westwood have reported the theft of dozens of GPS units during the past couple of months.

While many have been taken from cars parked in driveways or on residential streets, others are being stolen from hotel parking lots or office parks, police say.

The global positioning systems are "pretty easy to sell, and they have a good market value," said Russell Jenkins, deputy chief in Braintree. "People leave them on their dashboards, almost like an inviting target."

GPS units, global positioning systems for automobiles, can retail for $200 and higher. The devices often attach to a car's window or dashboard with a suction cup, making them highly visible and vulnerable.

GPS thieves seem to favor large lots, apparently seeing them as safer than driveways and offering multiple targets. And they have become smarter, say police. Even if the unit is tucked away in the glove compartment or center console, thieves detect its presence by noticing a round mark left on the inside of the windshield by the GPS suction cup.

Rick Wardell, the general manager of the Extended Stay hotel in Braintree, where five GPS units were stolen in seven car break-ins over the past couple of months, advises guests to use a little water and clean the circle off the windshield.

"They are quite savvy," he said of the thieves. Three men were recently arrested in connection with the thefts in Braintree and other towns.

In Westwood, 15 to 20 GPS units were stolen over the past six months, said Detective Paul Toland. Cars in office parks off Route 128, near the intersection of Route 109 and off the East Street exit, have been targeted.

"The thefts are ongoing, unfortunately," he said.

Foxborough was hit by thieves during June and July, said Police Chief Ed O'Leary.

"We had a series of them taking place at local business travelers' hotels," he said. More than a dozen GPS units were stolen, mostly from leased cars, he said. Police stepped up patrols in the area around the Comfort Inn and Marriott Courtyard, but no arrests were made.

"We had a rash of thefts over about two weeks in the summer," when about five were stolen, said Canton Lieutenant Helena Findlen, but it quieted down after that. The thefts were mainly from unlocked cars on residential streets, she said.

About 10 or 12 have been stolen in Mansfield, many from industrial areas and most recently from a Red Roof Inn, said Detective Sergeant Sam Thompson.

The thefts in Brockton have been smash-and-grabs, said Lieutenant John Crowley. "We've been averaging three a week for the last month. Prior to that we didn't have any."

For whatever reason, some towns have been spared. Detective Lieutenant Robert W. Wright knows the units are hot, but not in Marshfield. "We've been kind of lucky," he said.

Matt Carroll can be reached at mcarroll@globe.com.