In today's technology-rich environment, you don't need to be paranoid to feel like you're being watched. Sci-fi depictions of next-generation surveillance equipment have made it easy for many of us to fear our privacy is under assault. While an average life lived in an average way is of little note outside a person's inner circle, the capability to track a person's every move exists and this has consequences for average citizens and corporate raiders alike. Tracking a person's car can yield volumes of useful information, but fortunately tracking devices on vehicles can be detected. More sophisticated devices can be cleverly hidden, but there are steps you can take if you're concerned your vehicle is being tracked.
Consult a lawyer. If you truly believe someone is clandestinely tracking your movements, they are likely breaking the law. There are exceptions if the tracker is the owner of the vehicle, such as a co-owning spouse or an employer's vehicle such as a delivery truck. Employers are permitted by law to track company cars in most states, provided they are the registered owner. Laws vary dramatically by state, so be proactive and seek counsel.
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Conduct a visual inspection of the vehicle in question. Bring a witness when you conduct the sweep to ensure that you are not accused later of planting the device. Perhaps the lawyer you consulted is available. You can foil a tracker with this simple step. GPS devices need an eye to the sky, and while some are designed to operate inside buildings or in other low-signal environments, they won't work if they are too deeply buried in the car's nooks and crannies. Check the wheel wells, bumper recesses and the rear axle. Units are often held in place by a magnet, so focus on reasonably open metal areas.
Try another inspection in the dark if you fail to find the tracker at first glance. If you are unfamiliar with the look of your undercarriage, an innocuous looking black box might not stand out. Check again in a dark garage and look for any light source. Most GPS units have an LED indicating the remaining battery power.
Buy a radio frequency scanner. If the device is transmitting your whereabouts, a decent RF scanner with a broadband range will pick up the signal. You might need to disconnect your car's battery to limit noise. There will still be some residual noise from stored power in your car's memory, but it shouldn't light up as much as a tracking device will.
Passive devices that collect information, rather than transmit, will still show up on your scanner due to the power source if your scanner is sensative enough.