Privacy of what places you visit is already pretty much dead. We're the last generation who lived like that.
I'm not saying this is good or bad. Just that it is, and we have to adapt.
For anyone wondering how this works: the cellular modem is a separate general-purpose computer that runs code from the manufacturer and the service provider, the only thing needed to allow tracking a phone that's off is circuitry to allow the modem to draw power independent of the rest of the phone.
I guess a faraday pouch might be helpful, but I recall reading these aren't really as effective as many people believe.
I guess the gasket let enough EM through?
Amusingly, crumpled aluminum foil seems to have a better track record.
Same principle applies in the visual spectrum.