Unless he started wearing Lavellan's perfume himself...
I didn't even think of that, but now I remember how on historical tours the guides would always talk about how people in colonial times would be heavily perfumed and/or have scent bags (there's a name for this but I don't remember) because being around so many unwashed people all the time was sort of awful. So whereas now we see perfume/cologne as something people use only in certain instances, it was pretty much a necessity for anyone "back then".
It could even be what they eat...When I first started dating my now-boyfriend I was eating so much garlic that I smelled like it constantly. He even commented on it months later. Luckily he's Mediterranean so he likes garlic, but it was a little unsettling...
I do wonder how close Cassandra had to be to smell it on him though. Stay away from my elf Seeker!
Maybe not that close, wind and all.
That said though, bear with me a little bit as I go on impromptu scented oils and perfume rant (you can see pretty fast that this is an obsession of mine).
Going by the assumption that Lavellan perhaps wasn't wearing perfume as we know it (commercial designated perfume) but more a skin protectant product or medicated product kind of a deal (forest, heavy travelling through several regions, and Skyhold itself being up on an icy mountain can all lead to skin conditions like bug bites, scratches, dry, cracked skin from too cold air. To me it's just a must that Lavellan wears something on her skin because it's just painful otherwise. It's not even a matter of unnecessary personal maintainance, more like... healthcare when living in harsh conditions. She ain't living in comfy cities and apartments like us).
The chances of Lavellan wearing some kind of oil product instead of a butter/balm type product is significantly higher. Oil is easier to create, handle and preserve, and have multiple uses as treatment for burns, aches, pains, even headache or nausea (whereas back during middle age timeline, butter type product is the more expensive and frivolous of the two).
Scented oils are incredibly enduring (we are talking days here if she's in a very cold area and doesn't bath every day) and transfer very easily (skin). As said above, the most potent and enduring perfumes are the ones with the highest percentage of base oils in them (eau de parfume, esprit de parfume, and the strongest of them all, extrait). From the two romance cutscenes we have seen, it doesn't surprise me that Solas may have her scent on him (definitely enough body contact there with all that... heavy... petting... stuffs). Cassandra probably only needs to stand within 2-3 meters of him to catch a whiff with the right wind condition provided (Forbidden Oasis any one? Where the wind is so strong you can't walk outside the zone).
I've never had the perfume banter triggered for me through my 2 fem Lavellan playthrough but when I heard about it, it sounded very natural to me because... the same thing happened to me and my boyfriend. Once you get to a certain degree of physical intimacy (and I'm not talking sex here, rather when you become physically comfortable with each other. You no longer toe around, or feel unsure about boundaries. You can easily relax with the person in close proximity, hugging, holding, and generally staying right in each other's personal bubble, close enough for the scent to transfer), your scents start to mingle.
When I read about that banter, I literally went 'Ah, so they are that comfortable around each other now' (remember in the first and second romance cutscenes, you still get the feeling like Solas is still very reserved around Lavellan. He holds himself back a lot and stands out of arm's reach, hearing that banter is like confirmation that their relationship has progressed to that level).
... Rant end...
Ummm... somehow I feel a little embarassed having shared that... excuse me now as I go back to lurking