What would happen if there was an App to tell you who was carrying a gun near you?

[petronius]

150. No more an invasion of privacy than your hypothetical gun sensor, and likely

less so - I'd envision a BAC sensor as passively reading the chemical signal emitted into the environment through breath and evaporating sweat. If a person is actively giving off a signal, would it be privacy-invading for another person to passively detect that signal? (By contrast, your 'gun sensor' would likely need either an active scan of the carrier, or require that person to actively give off a radio signal or something for the sensor's convenience - both would be more invasive.)

So good question: is it an invasion of privacy to scan the contents of a person's pockets or purse? Is it more or less of an invasion to scan the chemical contents of the air they have given off?

(Of course, the use of any of this info could rapidly segue into the reprehensible, discriminatory, and/or unlawful, but that's not what we're talking about, right? This thread is just about measuring information in the local environment, and whether that touches anyone else's right to privacy...)