I didn't like those things because I don't like the police being able to arbitrarily pull people up and search them. I mean, I agree to be searched going onto an aeroplane or going into Lang Park, because those are the rules I agreed to abide by when purchasing my ticket. But just walking along?
Anyway, it seems that the 'reasonable suspicion' that causes the cops to search you is a reaction from a drug dog. However, it's instructive to read the Police PR release in its entirety:
Drug operation, Brunswick Street Railway Station: Police have arrested 26 people following a policing operation at the Brunswick Street Railway Station on Friday night and early Saturday morning. The operation involved a passive drug detection dog team as well as police from the Railway Squad, Public Safety Response Team, and the City Tactical Crime Squad. During the operation 80 people were detained and searched after the drug detection dog indicated that they had been in contact with dangerous drugs. Police arrested 26 people for 28 drug related offences as well as two other people for public nuisance and stealing. Those arrested will face court at a later date.Get that? 80 people searched, 26 had something on them - that's a 32.5% success rate.
I saw a documentary about drug sniffing dogs once - they said that a single false positive meant the dog was fired from the drug squad. Obviously not in Queensland.