Do dogs recognise smells of things they've never seen?

I definitely think they categorize things (which is how they think -- they dont' think in 'words' they think in 'frames' or pictures or ideas).  They may categorize things as generally as "sick" or "good" or "bad" (adding the human words) or "prey" or "fun" or such concepts.
 
My Foxy the Mostlie Sheltie (I lost him in March - he was about a month shy of 19) -- Foxy could sniff cancer.  Did he know what cancer was?  Of course not -- and yet HE alerted ME to cancer in one of my dogs years ago -- he started sniffing Muffin right in front of me -- (the tumor was in the prepuce area -- literally right under the tip of his penis) and when I chided him to 'leave your brother alone' he looked up at me finally with one of those Foxy "THEN ***YOU*** look at it!!!" looks. 
 
Later at the vets, he literally went to my vet and 'told' her in much the same way that a dog in the waiting room had cancer.  He knew we identified it as something bad in Muffin and he dragged Dr. D over to this dog and she sized it up immediately and said "Foxy I know -- we KNOW he is sick.  But you're absolutley right!"
 
She was pretty blown away by it -- the owner knew the dog was dying -- and so did Foxy. 
 
He definitely alerted ME to what he knew was a 'bad' smell.  A 'sick' smell if you will.  I could cite many other examples.

I know my hound can identify a "dead" smell.  When my older Yorkie died at home a couple of years ago, Maska took one sniff, walked to the middle of the floor and promptly vomited.  He is not a dog that vomits much - maybe twice in the 4+ years I've had him.

I think it's almost impossible for we mere humans to really understand WHAT information they get from scent.  We don't have a Jacobsen's organ and really don't understand what one DOES tell them simply because it's a sense organ that processes scent in a way we can't even imagine. 
 
Scent is a dog's primary 'sense' -- it's the strongest and it's hugely linked to that inate thing in a dog that proceses things on a very basic good/bad, fight/flight, goes-in-my-mouth/NOT, friend/foe thing -- it impacts nearly all of their decisions.  "Recognize" is probably a misleading term for we humans because we see it as a "known quantity" -- and with a dog it relates to that part of them that evaluates things purely on an inate dog level that kicks in a particular response FIRST, and then they 'think' past it. 
 
Like Foxy smelling that cancer -- I doubt he'd ever smelled it before -- but he knew it was a 'bad' smell and he got convinced it was something he should communicate TO ME.  (that was the decision part). 
 
Because he then knew I acted ON his warning, and he knew it grew in a few days and then it was removed (and I showed him it was and TOLD him it was what he warned me of) -- so later on when he encountered that scent it kicked in as something to report to a human.
 
But there are SO many factors involved -- and with the limited scent capabilities WE have, it's tough to figure the word "recognize" -- I doubt they 'recognize' as in "Oh that's a ____________ even tho I've never seen it" ... but they do 'recognize' good/bad/interesting/instinctive stuff.

Yes, I think something makes them react - or think about reacting. I know a lot of people with Border collies have remarked that, if they have the chance to go watch sheep, or a trial, or whatever, and don't bring their dogs (or all their dogs) - they notice the dogs get VERY excited by the smell of sheep manure on their person's clothes. Some people tell me that when they pull into my road and the window of the car is open, their dogs practically go out the window at the first whiff of sheepy air!

I think they can tell that something is "prey" - ie, some smells trigger that thing in their brain that says "chase/kill it". They don't go wild because they have higher processing ability that is able to tell them that the thing is not actually around to chase/kill. Or even if the thing were around, they might be unwilling to do something licensed by Mom/Dad, because they are well-trained (and not the type of dog that chase/kill overrules training, lol).

I think they have instinctive knowlege that tells them where to hunt animals with that type of smell, and how to bring them down, which may also be triggered by smell. Ie, they know whether they are looking for land animals, tree-dwellling animals, or birds.

ORIGINAL: brookcove

I think they have instinctive knowlege that tells them where to hunt animals with that type of smell, and how to bring them down, which may also be triggered by smell. Ie, they know whether they are looking for land animals, tree-dwellling animals, or birds.

That's what I'm trying to get at. Many types of animals have very distinctive smells. My feather collection smells like birds to me, and one of our cats goes bananas over it if she gets a whiff. She just wants to eat those feathers. She has smelt birds and seen them so before though, so she can make that connection that feathers smell like birds. But if  a dog or cat smelt feathers for the first time and had never been anywhere near a bird, would they know they were smelling a bird, or at least one of those things that flies around in the trees?

Reptiles have a very distinctive, musky odour. Snake bags smell pretty intense, even when they've been washed. Reptiles never struck me as a particularly strong smelling animal before, but once I'd smelt a snake bag I realised they they certainly do have a smell that doesn't smell like anything else. Similarly, herbivores all smell similar, but different herbivores still smell differently. When there are damp kangaroos around, even I can smell them. They don't smell like cows, even though they eat the same thing.

I find it all very interesting. Thanks for your responses![]

I have seen the herding dogs flip over fleeces; hunting puppies go on point in the backyard first time out; bassets and beagles crazily chase the trail of backyard rabbits.
 
I think this is one of the traits associated with instinct.  The more the lines have been bred with working dogs, the more likely it is to be present.

I am amazed that this came up because I have been mulling over the same thing...
 
Last week, Dodger went after a coyote, I know he's never smelled one before, the closest thing he's ever come across is a fox. I have no explanation as to HOW he knew it was prey and since he was within a few feet of it, why didn't he assume it was a dog (our last dog considered them dogs) - and since Dodger avoids dogs wherever possible, it obviously means he KNEW by the smell that it was prey. He picked up the scent about 50 meters away from where the coyote was hiding and sprinted in its direction - no hesitation.  He didn't start baying until it was within sight though.  Of course his breed has been used to hunt coyote's but certainly not in this area and since he's never been exposed to their smell, it's quite intriguing (although I nearly had a stroke).
 

even I as a puny-nosed human can classify smells into "rodent", "reptile", "bird", "large mammal", "fried foods", etc.. Have you ever gotten down on your hands and knees and tried smelling places where your dogs are sniffing? sometimes you can get a whiff of some interesting smells.

That's pretty amazing. I heard a similar story about a bird dog who was sitting near a caged budgie at an animal themed church service. Partway through the service, the bird dog suddenly realised there was a caged bird nearby, and it then suddenly dawned on it that it was a bird dog and that was a bird. It got very intense! I didn't get to speak to the owner, but I heard it hadn't ever encountered a caged bird before, and budgies are rather different to pheasant and other game birds. []