"Ever wish you had a tool that could prove your suspicions were valid?" Bluewater Security Professionals asks on its Web site (bluewatersecurityprofessionals.com). "Next time you sense suspicious activity with your vehicle, make sure (the $435 PTS Tracking System) goes along for the ride. The site includes "hints" on where to put the GPS antenna--mostly in places where it wouldn't be seen. But forewarned is forearmed: Here are some of the spots where a GPS unit is likely to be stashed.