Originally posted by Grimtempest:

I'll buy that i suppose since i've never thoroughly tested it and just had incidents of them spooking upwind for unknown reasons, i've often wondered why deer were the only mob that seemed to spoke based on upwind or not. Maybe other random environmentals caused it.

The rest i'm 100% sure of since i've spent alot more time fighting mobs then trying to sneak up on deer.

The devs didn't model smell in the game. They did model sound. Most creatures can see 20 meters in a 90 degree arc in front of them. Sounds travels 64 tiles and the creatures can hear it in a 360 degree radius.