Is when we can perceive a smell as part of spirit communication or in the residual energies of a place.

Some spirit communicators may send the medium a scent that identifies them, such as tobacco, aftershave, perfumes, bread, flowers etc.

It is quite random when and where the sense of smell becomes a part of mediumship. It often happens spontaneously when we least expect, it is a part of mental mediumship rather than all in the nose, like recalling a smell rather than actually smelling it under your nose. All the same it feels very real!
Sometimes though spirit can apport the smell for a group to sense at the same time.
A medium friend of mine passed away recently. On the night she passed before I discovered she was gone, I suddenly smelt a strong hospital smell – 3 others in the room reported the same as I was running my development circle on that night. I discovered later that evening she had passed away in the ICU of a local hospital, 3 hours earlier, and I believe it was her way of letting me know she was gone.