>If the Tor user has his phone somewhere nearby and if certain types of apps are on his phone, then his mobile device will ping back...
Surely the user would have had to approve microphone access for the app first, and it'd better have a good excuse.
Even then, does e.g. iOS allow backgrounded apps to listen in on the microphone ? Pretty sure only Siri has that level of privilege.
Has this whole ultrasound beacon thing taken off in the ad world ? Seems to (thankfully) require quite active user involvement to be able to work.