This would entirely depend on population density and how often people are in relatively crowded areas. Something like this may or may not work in an apartment b

This would entirely depend on population density and how often people are in relatively crowded areas. Something like this may or may not work in an apartment building, but it seems like it would definitely work on crowded city streets, particularly if you allow for a transmission delay as it spreads from one device to another. (And to that end, in a big city like Hong Kong, you probably wouldn't really need massive adoption, just a few % as you are in close proximity to hundreds of people fairly regularly).