I'm talking about an actual faraday bag, not a foil bag. That, combined with a powered off device keeps you fairly protected. They aren't going to get into your phone if it's powered off.

it also provides an alibi. "see, my phone was here all day long."

There's going to be a deviation in usage regardless, it's not providing an alibi. A gap of time when you're not using your device that you normally would be is a marker they look for. They key is keeping them locked out (again, power them off).

I mean, a proper Faraday cage will block anything unless the US government has figured out how to break the laws of physics...