Cell phones aren't allowed into classified environments in US government buildings because they can be remotely turned into listening devices without the owner's knowledge. There's been guidance on this for a long time. The cell phone in your pocket is definitely a surveillance device.
Let's be real. You don't keep your phone in a Faraday bag and you probably never will.
Unless the device is programmed to automatically enter into that mode in the absence of any signal on the assumption that if it is in a shielded location it is likely interesting.
Let's be real. You don't keep your phone in a Faraday bag and you probably never will.