It feels like that can be exploited in some ways. As a first thought it reduces the privacy of the reporting 3rd party phone. I.e. I can leave a fully charged phone in my wife’s car and track her for weeks while she will have the burden to recharge her phone for network/gps power.

A regular gps tracker would need much more energy.

Edit: another scenario, leave it in an isolated hut. If I get a signal, someone is close to the hut.

Edit 2: if I piggy back the protocol and can manipulate the key schedule (chose key A or B) then I can leak one bit of information through the third party phone. The third party phone may be allowed to communicate while my sender isn’t.