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I've heard that mobile phones send signals to the tower even when the mobile goes off so your location can be detected. Is this possible?

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Welcome to Skeptics! We want to focus our attention on doubtful claims that are widely held or are made by notable people. Please provide some references to places where this claim is being made. In particular, I'd like to see what people mean when they say "off". Not on a call? turned off? battery removed? –  Oddthinking Apr 12 '13 at 3:59
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it's certainly a doubtful claim, in fact it's a totally bogus claim UNLESS with "off" he means turning the screen off which leaves the device switched on and capable of receiving calls, which of course requires it to send keep-alive signals to the network which can be used to triangulate its position. –  jwenting Apr 12 '13 at 5:33
    
It is obviously a confusion of the sort alluded to by jwenting. Airlines require phones be turned off to stop their transmissions interfering with avionics. Phones with the display off are still on and are keeping in touch with base-stations so that they hand-over contact to the nearest one as they move, so that they can receive calls and messages - not primarily so that telcos can monitor your location. –  RedGrittyBrick Apr 12 '13 at 9:28
    
I have to say that I have heard this story too: you have to remove the battery for the cellphone to stop sending signals. Of course whoever was telling this factoid could never give me a real explanation for it... –  nico Apr 12 '13 at 16:43
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I know we can do better than this! So far, we have a far-fetched claim with no notability, arguments over what the claim really means, an accepted answer with no references at all, and answer to a different question, and an answer with a single, inappropriate reference. This is a broken window; let's fix it. –  Oddthinking Apr 15 '13 at 16:12

2 Answers 2

These articles state it can be done:

By September 2004, a new NSA technique enabled the agency to find cellphones even when they were turned off. JSOC troops called this “The Find,” and it gave them thousands of new targets, including members of a burgeoning al-Qaeda-sponsored insurgency in Iraq, according to members of the unit.

A secondary, ulta low power "baseband" processor remains "on" to listen to the cell tower. When the baseband processor detects an incoming call, it turns the rest of the phone back "on". Especially with older "feature phones", turning the phone "completely off" would sometimes leave the baseband processor still "on", thus allowing you to be tracked. For example, sometimes the phone had a timing circuit that will occasionally turn on the baseband to grab SMS messages every 10 minutes

On most Nokia phones, the alarm still rings even when the phone is turned "off."

A cellular telephone can be turned into a microphone and transmitter for the purpose of listening to conversations in the vicinity of the phone. This is done by transmitting to the cell phone a maintenance command on the control channel. This command places the cellular telephone in the "diagnostic mode." When this is done, conversations in the immediate area of the telephone can be monitored over the voice channel.

And the police can potentially push updates onto your phone that backdoor it and allow it to be turned into a microphone remotely, and do other stuff like that

Simply put, yes, it can be done. Technical details aside, your cell phone can be remotely powered on and off without either your consent or your knowledge.

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Need better sources on the ones that say the phone can't be tracked when off. The more reliable sources don't quite address the question. But I do recall having a Nokia alarm ringing after being switched "off". –  Muz Jul 29 '13 at 4:35
    
The Washington Post quotes aren't detailed enough to pin down what the sources mean by a phone that is "turned off". We all know that some phones keep talking to cell towers in modes that users typically think of as "off", but nothing in the sources you quote indicates that they can still be tracked when genuinely powered down. –  DJClayworth Sep 8 at 20:34

It can, if someone with the needed technology, like the FBI, wants it to and installs the needed device or software without your knowledge. This can be avoided by cutting off the power supply to the device, i.e. remove the battery from the device.

Cell phones can be taped and used as a listening device as long as their buttery is in, even if they are powered off. As this article by cnet demonstrates:

The surveillance technique came to light in an opinion published this week by U.S. District Judge Lewis Kaplan ...

Kaplan's opinion said that the eavesdropping technique "functioned whether the phone was powered on or off." Some handsets can't be fully powered down without removing the battery; for instance, some Nokia models will wake up when turned off if an alarm is set. While the Genovese crime family prosecution appears to be the first time a remote-eavesdropping mechanism has been used in a criminal case, the technique has been discussed in security circles for years.

Countermeasures against it are taken in the IDF (Israeli Army). I have experienced in first hand that to classes, briefings or meetings where confidential material is talked about or revealed, all participants must suppurate their phone from its buttery, and if your phone doesn't allow this, e.g., the Iphone, then you leave it outside of the room. Here is an article (in Hebrew) talking about information safety in the IDF that tells the same thing:

בדיונים חשובים משאירים [...] את הסלולרי אצל הפקידה בחוץ (בהתאם לפקודה האוסרת שימוש בטלפונים סלולריים, למעט "ורד הרים", במהלך אימונים, פעילות מבצעית ודיונים מסווגים)

Which translates to:

During important discussions, cellular phones are left with the secretary outside (With compliance with the order that prohibits the use of cellular phones, except for "mountain roses"*, during training, operations and classified discussions).

*: "Mountain Rose" is a secure IDF cellular network.

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Sorry to burst your bubble, but if you trace the sources the Kaplan opinion (and the cnet article) refers to a "listening device installed in the phone" which functioned whether the phone was on or off. In other words the phone was bugged - the phone itself didn't transmit. The other vulnerabilities were to either rogue software installed in the phone, or monitoring conversations. Such vulnerabilities easily explain the reason why phones need to be left outside secure environments. –  DJClayworth Apr 14 '13 at 21:52
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Also I believe the question was about location information, not voice monitoring. –  DJClayworth Apr 14 '13 at 22:04

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