For the past week I've been tracking my girlfriend through her mobile phone. I can see exactly where she is, at any time of day or night, within 150 yards, as long as her phone is on. It has been very interesting to find out about her day. Now I'm going to tell you how I did it.

First, though, I ought to point out, that my girlfriend is a journalist, that I had her permission ("in principle ...") and that this was all in the name of science, bagging a Pulitzer and paying the school fees. You have nothing to worry about, or at least not from me.

But back to business. First I had to get hold of her phone. It wasn't difficult. We live together and she has no reason not to trust me, so she often leaves it lying around. And, after all, I only needed it for five minutes.

I unplugged her phone and took it upstairs to register it on a website I had been told about. It looks as if the service is mainly for tracking stock and staff movements: the Guardian, rather sensibly, doesn't want me to tell you any more than that. I ticked the website's terms and conditions without reading them, put in my debit card details, and bought 25 GSM Credits for £5 plus vat.

Almost immediately, my girlfriend's phone vibrated with a new text message. "Ben Goldacre has requested to add you to their Buddy List! To accept, simply reply to this message with 'LOCATE'". I sent the requested reply. The phone vibrated again. A second text arrived: "WARNING: [this service] allows other people to know where you are. For your own safety make sure that you know who is locating you." I deleted both these text messages.

On the website, I see the familiar number in my list of "GSM devices" and I click "locate". A map appears of the area in which we live, with a person-shaped blob in the middle, roughly 100 yards from our home. The phone doesn't go off at all. There is no trace of what I'm doing on her phone. I can't quite believe my eyes: I knew that the police could do this, and telecommunications companies, but not any old random person with five minutes access to someone else's phone. I can't find anything in her mobile that could possibly let her know that I'm checking her location. As devious systems go, it's foolproof. I set up the website to track her at regular intervals, take a snapshot of her whereabouts automatically, every half hour, and plot her path on the map, so that I can view it at my leisure. It felt, I have to say, exceedingly wrong.

By the time my better half got home, I was so childishly over-excited that I managed to keep all of this secret for precisely 30 seconds. And to my disappointment, she wasn't even slightly freaked out. I don't know if that says good or bad things about our relationship and I wouldn't want you to come away thinking it's all a bit "Mr & Mrs Smith" around here. Having said that, we came up with at least five new uses for this technology between us in a few minutes, all far more sinister than anything I had managed to concoct on my own.

And that, for me, was the clincher. 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.