GPS is ubiquitous.  I use it to track my walks, runs, and bike rides.  It records my pace, total distance, altitude variations, and produces a graphic map of my completed route. I can take iPhone pictures along my route and tag points on the route to coincide with the picture.   I synchronize the data with a website that records all my events in an quickly accessible data base.  Of course, GPS is a new necessity for many people, increasing from about 1/2 million to 15 million in just 4 years.  All late model vehicles are equipped with the units.
So, what does this have to do with employment law?   Maybe the answer will come to you as you reflect on some micromanager in your career.  Maybe you can still hear one of his or her most used questions:  Where were you?  or Where have you been?  GPS provides the answer.
Employment law is of course concerned about privacy in the work place.  The right of privacy turns on the general idea of a person reasonably expecting to be left alone or unmonitored in certain situations.   The answer is never “one size fits all”.  Privacy is unique to each environment and each person’s role in that environment.  In the work environment, the employer can define the limits of your expectations of privacy by disclosing to you that your desk drawers or locker may be searched, or that your email may be monitored, or that internet usage is to be for business purposes only.  Most information workers are aware that their employer has an interest in protecting that information from dissemination, and that security precautions are installed to curtail theft or misuse of data.   The Sarbanes-Oxley Act for example requires precautions to be designed, installed, and policed to prevent insiders from profiteering from sensitive market information.
Before GPS, I remember the favorite ploy of employers looking for a reason to terminate an employee was to conduct an audit of his employment expense sheets.  With a few calls, a bill could eventually be found that was inflated or not clearly related to work.  Now, the employer can make a stronger and even more convincing case by using GPS tracking data to show an employee has lied about his whereabouts.  The reason is that GPS is not just real time, but historical.   Think of your iPhone, Blackberry, or your company vehicle’s GPS as an in flight “black box” that records a multitude of travel information.  To state the obvious, it tracks where you are.  The System also tracks where you were at all points on the route, your speed, your braking patterns, and your signaling, and yes, the times of day these events occurred.  Your movements can be real time or later monitored on a company computer.  If a car is part of your compensation, your “off-work” use of the vehicle will be monitored too as a practical matter because you can’t disable the factory equipped system.  Many computer applications now identify businesses (restaurants, bars, hotels) located within a tight radius.  Where you likely spent your free time may be discoverable with a few clicks by an intrusive employer. Don’t assume that because you rented a vehicle, your employer cannot obtain GPS tracking data from the rental company.  If your employer is paying the bill as the designated customer, while you are only the designated driver, the employer may obtain the tracking information from rental car company.
So, there is no bright line privacy protection test.  Generally, where you are paying all or part of the mobile phone bill or the rental car fees, or providing your own transportation in your own vehicle, and use the phone or vehicle for both business and personal use, you will have a good argument that at specific times of day and for specific personal uses, you privacy rights apply to refuse to answer employer inquiries of “where were you?”.   But the converse is true: when you’re on company time and the company is paying for the use of the phone or vehicle, you likely do not have an enforceable privacy expectation, especially if the employer has put you on written notice that your movements will be monitored.