That does work, unless you're someone like Snowden, who is targeted by the strong surveillance apparatus.
Do you believe >300M Americans already have malware on their phones which runs when they hit Airplane mode, and tricks them in to believing all radios are off, when it is in fact still pinging NSA servers with telemetry data?
I don't. Such technology surely exists, but it is not massively deployed, because doing so would spoil its usefulness. Basically, "We're not that important."
At WWDC Apple revealed that their “Car Keys” feature would work 5 hours after the phone has died, so you don’t lose the ability to access your car.
They didn’t announce new hardware that could do this. It’s available in every new iPhone. This is proof that this capability (to run software even when the phone is “off”) has been around at least for a number of years. It’s not a huge leap to imagine that some malware could rewrite the firmware and enable e.g. microphone listening when the phone is off.
Do you believe >300M Americans already have malware on their phones which runs when they hit Airplane mode, and tricks them in to believing all radios are off, when it is in fact still pinging NSA servers with telemetry data?
I don't. Such technology surely exists, but it is not massively deployed, because doing so would spoil its usefulness. Basically, "We're not that important."