they teach the dogs to false alert,, ex cop barry cooper did some youtube videos about how he trained his dog to false alert so he could gain legal entry to cars etc,, he makes a living now making "how to not get busted" video tapes, he explains everything though a cops point of view and how to counter act the police, interesting stuff.
here is the video, its pretty long,, it shows how he trained his dog to false alert
Barry Cooper 's Never get Busted Again Full version
https://www.youtube.com/watch?v=ZyAjLkBCWKI