Your mobile phone company could make money from selling information about your location to the companies that offer this service. If you have any reason to suspect that your phone might have been out of your sight, even for five minutes, and there is anyone who might want to track you: call your phone company and ask it to find out if there is a trace on your phone. Anybody could be watching you. It could be me.
Read this Guardian Unlimitied article entitled “How I Stalked My girlfriend”. Watch out: there is a website out there (the author does not mention it) that tracks a cellphone’s whereabouts.