Such devices aren't hard to come by, either. A quick Google search for "GPS spouse tracking" yields several gadgets and methods that can perform this type of dirty work. Some of these devices are so inconspicuous that they are difficult to detect; the GPS Tracking Key, for instance, comes equipped with a powerful magnet so the user can attach it to the bottom of a car in a matter of seconds.
Some private investigators use GPS devices to track people for their clients. The Gomez Detective Agency, made famous through the TV show Cheaters, lists GPS as one of the many surveillance tools it uses to catch people with their pants down. The agency's investigators follow spouses from place to place and videotape evidence of infidelity, enabling the cheatee to catch the cheater in the act.
Rental car companies can legally track you and impose surcharges for violating their terms of service, as Ron Lee found out after he crossed the Nevada state line in a car that he rented in California. When he returned the car to Pay Less rental car company, the company socked him with a $1400 additional charge. So be sure to read the fine print--or at least ask about jurisdictional or mileage limitations when you get a vehicle.
Next: Smartphone betrayal and revealing applications.
Some tracking programs, like the pay-as-you-go AccuTracking software from AccuTracking Inc, can be downloaded to a cell phone. After activating the software, the tracker can determine the location of the phone at any moment by logging in online. The scary thing about AccuTracking is that the person using it can run it in "covert" mode, which renders it undetectable by anyone who doesn't already know it's there. AccuTracking forums member hsp2072 voiced his suspicion on the site's forums, saying that his ex-wife had given him a cell phone as a Father's Day gift and that he suspected "she had this tracking stuff installed in it before she gave it to me." Yikes.
Someone could take a cell phone, install the program, and give it back to the owner without leaving any sign that the software had been downloaded and activated, as London Guardian columnist Dr. Ben Goldacre proved when he tested the software out on his girlfriend's phone. He had easy online access to her every location, in real time.