When Austin Sweazy pulled into the parking lot of an auto-repair shop, his fiancee reminded him that even though their errand inside would last just a few minutes, it would be smart to lock the car doors.

After all, she said, “you don’t want anybody to steal your GPS.”

Sweazy heeded her advice. But it didn’t matter. In the few minutes the couple were inside the store, a thief smashed the window of Sweazy’s car, snatched his $600 TomTom portable navigation unit off the windshield and fled into the gathering dusk.

Valuable stuff has been swiped from cars forever, but the theft of portable satellite-navigation units is dramatically increasing in many places. Crime analysts blame an alignment of economic and technological factors, while victims lament that the units, which cost several hundred dollars, are rarely recovered or replenished by insurance.

In Maryland’s Montgomery County, outside Washington, D.C., 620 portable navigation devices were filched from cars through Aug. 31, blowing past the 189 taken in all of 2006. In downtown Philadelphia, GPS thefts jumped to 88 in the first eight months of the year from 33 in the same period of 2006.

Police in San Francisco and the Boston area also have cited increases — as have authorities in Australia and Britain.