I found this one very interesting, about using ultrasound for tracking https://media.ccc.de/v/33c3-8336-talking_behind_your_back#video&t=3 https://media.ccc.de

I found this one very interesting, about using ultrasound for tracking https://media.ccc.de/v/33c3-8336-talking_behind_your_back#video&t=3 https://media.ccc.de/v/33c3-8336-talking_behind_your_back#vi... I think it was also on HN a few days ago..