Not only do dogs have a crazy sense of smell, but I find most dogs that I come across also have an amazing sense of discernment. They not only seem to know a mood in a room, but also when someone new comes over, (perhaps by their posture), a dog can tell their character.
I believe that "bad" people put off a specific smell. Not sure what exactly it's from. When Strider was a puppy I did a lot of drive building exercises from SAR exercises. He's not a SAR dog, but he's really good at air scenting and tracking. When he
feels like it. lol.
Once in a while he'll catch a scent, and he just behaves a particular way. I know it's not a critter. I know it's not a lost kid or anybody innocent. He only alerts that way if it's the smell of someone doing something bad.
The first time he alerted like that, I let him follow the track and he found a huge stash of stolen AR-15 ammo and crescent clips in a giant cinderblock waaay out in the middle of a field, wrapped in an old coat. The police were overjoyed, and said they'd recovered the stolen rifles but never thought they'd find the missing ammo.
The second time, he led us a couple of miles off the trail in the middle of the woods and recovered a stolen, brand new, $1200 road bike buried in the leaves.
The third time he alerted like that was at a trail next to some railroad tracks in Interbay, in Seattle which led to a homeless camp. I did NOT let him go there though, as I was by myself and honestly did not want to find a dead body or a nest of armed criminals.
It's just odd how he hones in on bad people when he alerts and tracks, and it's purely by scent.
There were also dogs on the SAR team that were somehow able to hone in on the scent of the lost person and ignore the smells of other hikers, other searchers, law enforcement, etc. A lot of dogs weren't able to do that, but that fact that some can discern the scent of a distressed person is pretty interesting. It would be really neat if we could isolate the specific smell and use that for training, sort of like how we can use stuff like cadaverine.