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An LAPD helicopter claimed to have ID'ed protesters from above and threatened to "come to your house"
(www.motherjones.com)
This is a most excellent place for technology news and articles.
I wouldn't trust a foil bag to do anything 100%.
the only 100% way to not be caught is not bring it.
it also provides an alibi. "see, my phone was here all day long."
I mean, a proper Faraday cage will block anything unless the US government has figured out how to break the laws of physics...
Faraday cage does not prevent gyroscopes and accelerometers from working. Then if you remove it from a faraday cage in the future, it will transmit all that movement
Without GPS or tower based error correction any location prediction based on conservation of momentum in the phone will be useless before very long if the phone is moving.