I should have specified it but I considered that the be part of return by death.
Maybe only beast can smell it. We never had a background on Betty so maybe she is a half something. I mean she did imply she was older than she looks. But then Ram should have notice the smell. Or it’s just scenario smell. I guess there will be more explanation later on.
Ram has no horn and thus is much weaker with respect to magic, which kinda goes with my idea that you need some magic ability to smell it.But then again, Roswald has to know, and I assume there would be people with magical abilities as well in the castle from last book… Oh well, possibly scenario smell.
Yeah but we know Roswald has a tendency to not tell everything so it is possible. I just don’t get why if it’s working with magic, no one in the castle realised it.(Felix for example). They pretty much all hate the Witch so… again how many of them know what the witch smells. That could be the thing. Maybe they smell it but have no idea what the smell is and only think hat Subaru smells bad. That could work.
This is just my hypothesis, but what if it’s not really a smell? The only one who said they smell it was Rem. What if it involves different senses for every person? Like for Rem it’s a scent, but maybe Roswaal sees an aura? It is something that can be detected, especially by mabeasts (who is implied were created by the witch), but what if it’s not really a smell?
beatrice also spoke of the witch’s scent though
I still have no idea 
Just people with more foreknowledge than me smiling condescendingly in the general direction of my aimless guessing.