Oxygen is toxic to anything not adapted to it, just like chlorine and fluorine are to us. If they aren’t used to the oxygen our air it probably not only smells bad but is rapidly fatal at normal concentrations.
Just exactly right.
There is no selective pressure to develop the receptors and to devote the brain real estate to detecting the presence or absence of something that is always the same (different than if its amount varies at least, the variation has been of some significance to actions made, and ultimately to better “fitness” reproductively).
And there is pressure to notice novelty, to notice that the concentration of certain potentially salient substances that do sometimes change has suddenly changed.
You know how laundry detergent manufacturers like to claim they’re using a scent that mimics the scent of clothes dried outside on a line. What is that scent of clothes dried outdoors? Like the scent of outside air that you can notice when someone comes in from outside.
Sounds like a variation of the "If a tree falls in a forest … " thing. What if someone was around but they are deaf? Does it make a sound?
What if something is producing molecules that we are used to? Does it make a smell?
Like the tree falling thing, my answer is “Of course!”
Take a person and isolate them in an ultra-clean room for a week. Then let them go outside. They’ll smell stuff others around them won’t.