A lot of people are a few inches from my pocket every day. And I can easily imagine software listening for such sonars and sending back fake responses. I think debarshri has a valid point.
> A lot of people are a few inches from my pocket every day.
Really? And your phone is on? With the screen visible for them to see what they are interacting with. And as they are fumbling around in your personal space inches from your hand or pocket, no one notices.
Get real! If true you should be more worried about pickpockets than some random gestures.