Can someone trace where you were a year or two earlier, based on your cellphone? If so, would you have to have been making a call? Or just have your phone on? Or can your past be traced even if your phone is off?

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    So the non-answer is it depends. All Cellular Providers keep logs of which phones were in range of their towers. It does not matter if you made a call; if the Cellular radio was on it was silently interacting with cell towers. But for how long does a Provider keep those logs? (it varies by Provider). If your phone's Cellular radio was off, it should be impossible for the cell towers to detect it (though a NSA briefing implies otherwise). And what if some spyware on your phone recorded your GPS track, and sent the log out when you turned on the phone. Just depends how paranoid you want to be. – cybermike Apr 9 '15 at 0:30
  • Whilst this data undoubtedly exists, making use of it is another matter. The US govt were recording all sorts of phone data prior to 911 for example but it didn't do them any good whatsoever. – Julian Knight Apr 9 '15 at 16:22

If you're an android user and this "someone" has access to your Google account the answer is yes, with Google location history:


If the threat is a law enforcement or intelligence agency you can be also triangulated passively as your phone "pings" cell towers consistently even when you're not making a call. As @cybermike said, its not clear how long they keep this kind of data but if Google can it's pretty plausible that they can as well...

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