I think your theory is pretty dead on. Deer learn to associate certain things with danage. A deer in the wild knows that human scent equal bad news, so they avoid that location when that scent is detected. The only curve to this is when you look at deer in a park where hunting is not allowed. However, if hunting is not allowed, then the deer have never had to associate human scent with danager. I know when I used to farm, it was very common to have deer walk right up to you while bailing/mowing/raking hay, combining corn or beans. I guess they don't view the scent of the exhaust as a treat, which would explain why so many of them end up in ditches, or worse yet, in someone's front seat.