Feature phones used to power up the baseband periodically, which I’m guessing is why you think that. People don’t really use those, anymore.
Yes, the NSA deployed a method of tracking phones that appear off. They can do this because GSM phones accept baseband firmware updates from the network without authentication. So, if you have an agent at the cell phone company, or if you can get your target with a stinger tower, then it’s possible to reprogram the phone to keep its baseband on even when it appears to be switched off. I was pointing out that I don’t think that’s possible in Cohen’s case.
