In one story, "The Crooked Man," Holmes called on Watson (then married and in medical practice) at his home. Holmes remarked "Hum! You still smoke the Arcadia mixture of your bachelor days, then! There's no mistaking that fluffy ash upon your coat."
Since Holmes was in the habit of keeping his pipe tobacco in the toe of a Persian slipper, there's no end to what interesting flavors and aromas the tobacco could have picked up-- assuming it was a used slipper. A sweaty foot topping might impart what could be considered a "foul aroma."
