Awareness is not sharp senses but the habit of using them — the sentry who registers that the birds have stopped, the traveller who catches the wrong kind of quiet. It is the passive, standing alertness a character carries about with them, and it is what the world is rolled against when something is there to be noticed.
Where a sense is dulled or the surroundings work against it, the test is penalised or refused outright: sight is at −20 in dim light and impossible in true dark, while hearing in the same place may be untouched. The GM decides which senses a given modifier reaches.
Concentrating or merely vigilant. A character deliberately attending to something — a guard peering down one particular alley — may test the better of Awareness or Perception. A character who is only generally watchful, or who is distracted, stressed or busy, must test Awareness. That is the whole of the distinction: Perception is what the senses can do when pointed, Awareness is what they do unpointed.
Most Awareness tests are unopposed and simply ask whether the character notices. Against someone actively hiding or stalking, the test becomes an opposed one against Stealth, and it is likewise what alert targets use to see through a physical disguise.
Smell. Nobody develops a discriminating nose in the abstract; it comes with a trade. A character distracted or unfocused tests their highest olfactory skill — Brewing, Cookery, Embalming, Herblore or Perfumery — to register an odour at all. Deliberately analysing a smell may be tested as Perception instead, but the detail such a test yields is far coarser than what the trained nose would have drawn out. Many beasts smell incomparably better than folk do, and are handled by their own rules.