A dog searching for a lost child is typically given an item of clothing to smell. But what does that scent “look” like?

To find out, scientists tested 48 dogs, half of which had special police or rescue training. In a laboratory room, the scientists slid each dog’s favorite toy across the floor to a hiding place, while the dog waited in another room. One researcher then brought the dog to the testing room and pointed at the starting point of the odor trail and told the dog, “Look for it! Bring it!” In one trial, the dog found either its favored toy or a different item.

Many of the surprised dogs continued searching for the toy used to lay the scent trail—an indication that they had a mental representation of what they expected to find. Bot ...