This trick works because on my phone, bluetooth is set to "visible". You don't have to do that in order to use bluetooth, but without visibility, you need a connection attempt to find out who's there and who isn't, and the distance to the bluetooth dongle within which a good connection can be made is usually smaller that for scanning.

Your supermarket would probably more easily scan and recognise your face, just like the people at the supermarket have always been able to.