So, you're a master criminal, or perhaps a cheating spouse, but either way you've covered your tracks and have a high court judge ready to confirm your alibi - you were eating dinner in a club when the deed occurred. Tickets paid in cash, and a hoodie to hide from the CCTV, your story is safe - except your mobile phone network knows exactly where you were, and when. But they're not going to tell anyone, are they?
Except they might. Network operators are bound by the Regulation of Investigatory Powers Act (RIPA), which requires them to store not only the details of every call you make and receive, but also where you were at the time. They keep this information for 12 months and make it available to the authorities on demand.
Of course, unless your spouse is very well placed they're unlikely to be able to call upon the RIPA to help, but if they suspect you in advance, your location might not be as secret as you think.
What does the network know?
When switched on, your mobile phone is logged onto the nearest cell site, which is recorded on servers at the network operator's data centre. The cell might not be the nearest physically, though in general it will be. What's important is the strength of signal from the handset's perspective.
In town the cell might cover an area as small as 100 yards across, but in the countryside they can easily cover ten miles. The deciding factor is generally the capacity of the cell, rather than range of the radio - so if you want to stay hidden keep away from places where people use their mobiles a lot, so cells will be dispersed.
In addition to the cell your phone is logged onto, the network operator can record your rough distance and the direction from it.
If you have signed up, or been signed up, to any kind of commercial tracking service, then external systems can connect to the network operator's computers and get that information.
According to the industry code of practice you should be getting random SMS messages reminding you that you could be tracked at any time, but those aren't always as frequent as they're supposed to be.
The networks make great play of the difference between where you are and where you were. They are perfectly happy to tell commercial services where you are, on demand, but they're not going to disclose your previous whereabouts without a RIPA request and accompanying purchase order.
To be sure your network isn't sharing your location data, change your privacy settings, which should protect you from the majority of commercial tracking solutions.