I have seen cadaver dogs in action twice.
The first time the dog was actually very good. He walked around for a minute, with the handler and then kept going back to the same spot. The handler took him back to the vehicle and they moved something, I think it was a lawn chair, over the spot that he had been going to and when they brought him back to the scene, he went right back to the same spot.
The second time, the dog hit on areas all over the scene, he covered almost a full mile and hit over a dozen times. The handler took him out of the area and brought him back and he didn't hit any of the same places that he had before. THey brought in a second dog and it hit on three or four of the places that the other dog had.
In the first case, the dog was right on, in the second they never found anything. Doesn't mean it was never there, it just wasn't by the time they started digging.
In short, I think that it depends on the dog some days. I hope that the ones that searched the car were wrong.