From D&D Wiki
 Changes From the Original SRD
Added (from Abilities and Conditions):
- " extraordinary"
- "Water, particularly running water, ruins a trail for air-breathing creatures. Water-breathing creatures that have the scent ability, however, can use it in the water easily.
- "False, powerful odors can easily mask other scents. The presence of such an odor completely spoils the ability to properly detect or identify creatures, and the base Survival DC to track becomes 20 rather than 10."
Is there any Feat that someone may know of, SRD or Homebrew, that allows a non-beast character to use Scent? It seems as though it could be worked for a decent Skill, especially for stealth-based characters or more so "feral"-like Races such as Orcs.
Edit: In The Book of Nine Swords p. 88 there's a stance called Hunter's Sense which grants you the scent ability. It's part of the Tiger Claw discipline.