Strong House of X vibes going on here. The Phalanx being unable to assimilate mutants, but being able to assimilate their evolutionary conquerors*, is such an interesting concept.
*I say that for lack of a better term, since the post-humans and human-cyborg race aren't really their successors but their replacements, and the ones who end the mutant line of evolution.
Wolverine has some absolutely crazy smelling feats, like backtracking a bear across hundreds of miles of wilderness (IIRC) after killing it in order to find the hunter that had driven it insane by using poisoned arrows and recognizing identifying Katie Powers' siblings after their faces had been twisted by Masque and their minds had been scrambled by another Morlock by smelling their relation to Katie despite the fact that they were in a a massive crowd of Morlocks in the middle of the New York Sewers.
I'm comfortable with saying that his nose's feats combined with his human intelligence and his ninja/super-spy/X-Men training would make him incomparably better at detecting alien invaders than a crowd of dogs who have probably never met an alien before.
I fail to see how, they didn't do anything but notice a new arrival then forget about it, they don't even whine to their keeper about it.
The point is that it's not supposed to smell like some alien at all, if it's in assimilated shape blending with something else, down to replicating existing health conditions.
In addition to what others have noted about the dogs actually noticing something weird about it, my point is more that Logan's a lot better at sniffing than a dog is and is much more likely to detect something off about a creature that is A) actually alien (even if they were actually able to hide their scent enough to actually fool regular dogs), and B) probably undergoing a bunch of weird chemical reactions internally if they are ready to become body horror at a moment's notice. Hell, in the latter case, his superhuman sense of hearing might detect the sound of weird construction slorping around a victim's guts.
And I'm STILL scratching my head at this take. The only thing they notice is something new entering their field of vision, which prompt them to shoot a glance and then... go back to minding their own business and not behave any differently until the Thing willingly chose to reveal itself to them some time later. THAT's when the dogs freaked out and knew something wrong was up. That's... just the movie at this point.
I don't have anything to say about Logan not being able to sniff the Thing underdoing any kind of shapeshifting activity, if it was aiming for pray and Logan was around but that's just a)moot for this thread and b)seriously, if this was some urban environment and he knew the Thing was around, Logan would be smart enough to know that instead of indulging hunting instincts he should just call SHIELD or the F4 for some proper quarantining ops.
Also, unless it's a perfect scent, the thing will smell funny in comparison to human beings.
https://www.bbc.com/news/uk-scotland-47627179
There's this lady that can smell parkenson disease and she's got nowhere near the fine tuned sniffer that wolvie has.
Wolverine is often surly because he knows what _everything_ smells like.
to say his other best friend must be a real pain to hang out when it's bright and he's transformed (colossus would be able to be seen from orbit).
One of his sidekicks literally shoots fireworks.