Individual molecules of air travel at speeds in the hundreds of meters per second (https://pages.mtu.edu/~suits/SpeedofSound.html https://pages.mtu.edu/~suits/S

Individual molecules of air travel at speeds in the hundreds of meters per second (https://pages.mtu.edu/~suits/SpeedofSound.html https://pages.mtu.edu/~suits/SpeedofSound.html). So it seems quite possible that individual molecules may be found quite far away from their origin quite quickly, even if the bulk is much slower in mixing (I'm not an expert here and would love to hear more if someone knows more about this). It sounds like this substance is bad enough that a couple of molecules per Litre would make the air smell awful, which together with the previous point may explain how the bad smell could be detected at such a distance so quickly.