In short, the police powers of stop and search are found in the section 1 of the Police and Crimanal Evidence Act 1984, where s1(3) PACE 1984 provides,
"This section does not give a constable power to search a person or vehicle or anything in or on a vehicle unless he has reasonable grounds for suspecting that he will find stolen or prohibited articles"When police use sniffer dogs as a basis for stopping and searching people is this lawful?
It depends upon whether or not the sniffer dog behaviour gives the officer reasonable grounds.
We can only answer that question if we know the (i) false positive rate, (ii) false negative rate, and how the Clever Hans effect is prevented.
The Clever Hans effect being a mechanism whereby the animal's behaviour is a result of unwitting cues given to it by its trainer.
Well, it seems that someone has decided to bother investigating the Clever Hans effect with regard to sniffer dogs - here's a link to the Mind Hack's article that has more details, Sniffing Out the Unconscious.
Link to original article dx.doi.org/10.1007/s10071-010-0373-2.
My previous notes,