full story at link: http://www.wired.com/threatlevel/2011/05/gps/

Kathy Thomas knew she was under surveillance. The animal rights and environmental activist had been trailed daily by cops over several months, and had even been stopped on occasion by police and FBI agents.

But when the surveillance seemed to halt suddenly in mid-2005 after she confronted one of the agents, she thought it was all over. Months went by without a peep from the FBI surveillance teams that had been tracking her in undercover vehicles and helicopters. That's when it occurred to her to check her car.

Rumors had been swirling among activists that the FBI might be using GPS to track them -- two activists in Colorado discovered mysterious devices attached to their car bumpers in 2003 -- so Thomas (a pseudonym) went out to the vehicle in a frenzy and ran her hands beneath the rear bumper. She was only half-surprised to find a small electronic device and foot-long battery wand secured to her metal fender with industrial-strength magnets.

'I think I must have found it right after they put it on, because there was no grime on it at all,' she told Threat Level recently.