Realistically speaking with another phone in front and imagining how close one has to be for it to work reliably (as it is not a 64x64 pixels image) I think it

Realistically speaking with another phone in front and imagining how close one has to be for it to work reliably (as it is not a 64x64 pixels image) I think it would be really hard not to notice. The same way you whisper to say something secret in a public space I would expect people who transfer something private in public would have the presence of mind not let other snoop on and react accordingly.