Danville, California police tracked down a stolen Subaru Legacy wagon by tracking the signal of the FasTrak toll payment transponder. They caught the thief who had snatched Dinah Thompson's car right out of her driveway in the early morning hours of September 22. The thief forgot to remove the transponder, and Thompson noticed on she was being billed for using toll road regularly.

She notified police who were able to recover the vehicle after three weeks and 4000 miles using a series of photographs taken at a toll booth. The Bay Area Toll Authority says it only turns over tracking information with a valid court order, except for tracking drivers to measure congestion on other roads.

But as FasTrak use in the Bay Area rises, the idea of government agencies observing movements stirs debate between crime victims who simply want to catch a thief and privacy advocates who fear an invasion into the private lives of civilians. "You always have to have a balance of competing interests," said John Soma, executive director of the Privacy Foundation at the University of Denver. "If there was a legitimate complaint of a stolen car, then the actions seem reasonable," Soma said. "However, if they start crossing the line and start randomly tracking individuals, then I think you have to go back to the classic justification and have probable cause."