Let's dig out our knowledge about smell first:
Metals don't smell in themselves. They smell because of what sticks to them:
The compounds people smell are actually aldehydes and ketones, and not any iron-containing compounds.1
Our noses are just not made to smell metal ions in themselves. But Adamantium is an alloy, right? It is a hellhole of Steel, Vibranium, and Unobtanium2. But it still does not emit molecules that actually can be smelled as our nose is a chemical analyzation facility.
So, Klaw can't smell the metal itself because (pure & uncontaminated) metal does not smell to the human nose.
Smelling Vibranium specific organic smear on the surface
Chemistry, biology, and physics established that you can't smell something that doesn't emit some kind of molecule that can be analyzed by our nose. But we do know some things about Adamantium that can help in establishing how it might or might not smell:
- it is rare3 and an alloy containing Vibranium.
- Depending on the writer, Adamantium is poisonous3 4 5
Most alloys share effects with their mother compounds, especially when it comes to toxicity. As Adamantium is poisonous, so should be Vibranium, unless the Unobtanium2 is.
So, Vibranium is a somewhat poisonous metal, as is lead or arsenic? That indeed can be part of the solution! There are bacteria living in high arsenic surroundings6 while others ingest lead7. The chemical compounds metabolized by these bacteria might or might not contain the actual arsenic or lead, but the presence of these metals in their surroundings do make these chemical compounds very specific. And these compounds are something the olfactoric sense can analyze.
So I propose that Klaw does not smell the actual Vibranium, but the (residue) of some bacteria that is capable of (temporarily) living on it and creating a specific mix of aldehydes and ketones that does not exist anywhere else.
Klaw the Super Nose
There is one alternative though: That would be that Klaw's nose is not like any humans in a way that he could possibly smell the chemical composition of metals and alloys. Like, he might have a gas chromatograph for a nose. But that still would need metal ions in the air to smell, and we know that Vibranium isn't that kinda metal usually as it is hard to work with. Also, there is no indication in other sources I am aware of, that give Klaw this sort of super-smell.
Or... Klaw didn't smell the metal at all. He realized what it was from the visuals and did a sniff just for the effects, claiming that he smelled it for the fun of it, as Broklynite pointed out.
1 - https://phys.org/news/2006-11-metal.html
2 - or whatever the current writer wants it to have in it
3 - http://time.com/4685052/logan-wolverine-adamantium/
4 - https://www.cbr.com/wolverine-adamantium-poisoning/
5 - https://www.reddit.com/r/AskScienceFiction/comments/656hux/xmen_how_can_adamantium_be_poisoning_wolverine_if/
6 - https://www.nature.com/news/arsenic-loving-bacterium-needs-phosphorus-after-all-1.10971
7 - https://www.ncbi.nlm.nih.gov/pubmed/29736824