Smells are stray molecules, we detect them only when they’re sufficiently concentrated in the air around us, when we don’t sense them anymore it’s just because the molecules are so few and far between that we’re unable to percieve their presence. As far as smell eventually going away, that’s bollocks, you obviously never had to use a medieval toilet. One of those old ones which don’t flush. The only reason the smell doesn’t linger in our bathrooms is that no object in a bathroom (With a few exceptions, like towels) is porose. And only porose objects can retain smells. Yes, this means that when yousmell shit it’s because small shit molecules are getting into your nose.