I know for a fact now that hounddogs track their prey, not just by smelling the animal's scent, but by smelling the animals fear.
I have seen on numerous occasions, Ray walk right by Moonie (and walk right into Moonie) when she is sitting placidly on the back of the couch or on the floor nearby. But if Moonie walks into the room, sees the blind dog, and then aprehensively slinks off, Ray's head will go up and his nose will start twitching. When Hugo, our scaredey-cat is twitching nervously anywhere nearby, Ray instantly goes into hunt mode.
When we are at the dogpark, Ray only hounds those dogs that are obviously nervous. Ususally it's the little dogs or young dogs but sometimes it's just a newbie that's never been to a dogpark before (and probably will never come again after having Ray blast away in it's face). If no nervous dogs are around, Ray will settle on dogs that are chasing balls or frisbees.
But the reason I am absolutley convinced that Ray smells fear and not just a generic animal smell is this. The other evening, Ray was asleep in his bed in front of the fireplace. I was giving Moonie a pill (a particularly foul-tasting thing if Moonie's reaction is anything to judge by) in another room. The second Moonie realized what I going to do, Ray got up off his bed, and headed our way, his nose twitching like crazy. Moonie hadn't made a sound and I wasn't holding a steak. It's the smell of fear that drives this blind hounddog.