So there were some questions I thought of this morning, and when I got home I did a quick search before posting. I will confess to skimming instead of reading every post. Here is what I think I know. For the sake of simplicity, I will refer to creatures with scent as animals.
1. Given the presence of a creature with an odor within the appropriate range, an animal will always act in a surprise round without rolling Perception. The animal can still be surprised by the rare odorless monster and by those that make an attack from outside of scent's range.
2. An animal - most likely an animal companion - does not automatically confer the presense of danger to allies. When seconds matter, even the AC's master will still have to roll Perception. Given some extra time, the presence of danger could be communicated by the animal, or its allies could pick up on its behavior.
3. The animal smells something and gets to act in the surprise round, but since scent does not automatically pinpoint a creature's location this means that a hidden enemy could still have total concealment against the animal. The animal can thereafter use a move action to locate the enemy.
So here are some questions.
1. Sometimes an animal companion and its master develop an empathic link. When the animal companion smells something and feels threatened, could that automatically alert the master and negate surprise?
2. Does having scent and the Lookout teamwork feat end up being as good as it sounds? Judging by the wording, it sounds like your chances of getting a full-round action in a surprise round improve.