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?
These articles state it can be done:
- Washington Post - NSA growth fueled by need to target terrorists
- The Atlantic Wire - How the NSA Is Using Cell Phone Data to Drone Civilians (In Pakistan)
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.
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.