I have a question about scent in the game. My 1/2Orc uses the 'Keen Scent' feat which allows the character to have the ability to use scent as a character feature in the campaign. From the description below I am always asked to make perception rolls to sense people/creatures/etc when using scent. I interpret scent to be automatic within the ranges given below unless the person/creature/etc is deliberately trying to avoid detection by masking their scent. Therefore, just as it is obvious to walk into a room and see the obvious furniture etc visually before you unless deliberately hidden or concealed it should be just as obvious to notice the presence of the smell of the furniture etc in a room. This translates that if you walk into a room with three unconcealed people you will see them so by similar idea you would scent the same three people. If they were visually concealed then a perception roll would be necessary to see them and if they were scent concealed they would also need a perception roll to discern their scent. However most creatures do not attempt to conceal their scent and therefore should be detectable by scent as an unconcealed person would be visually (ie. without a perception roll but automatic within scent range as given below). Unless the person/creature/etc is deliberately masking their scent there is no opposed stealth check to make. A rogue hiding in shadows is trying to conceal themselves visually and audibly not olfactory. Unless they carry or cover themselves in false scents or overpowering smells such as pepper, etc. I do not see why a perception roll is necessary and if it is what is the purpose or use of being able to have the scent ability.
"Scent
This extraordinary ability lets a creature detect approaching
enemies, sniff out hidden foes, and track by sense of smell.
A creature with the scent ability can detect opponents by
sense of smell, generally within 30 feet. If the opponent is
upwind, the range is 60 feet. If it is downwind, the range is
15 feet. Strong scents, such as smoke or rotting garbage, can
be detected at twice the ranges noted above. Overpowering
scents, such as skunk musk or troglodyte stench, can be
detected at three times these ranges.
The creature detects another creature’s presence but not
its specific location. Noting the direction of the scent is a
move action. If the creature moves within 5 feet (1 square)
of the scent’s source, the creature can pinpoint the area
that the source occupies, even if it cannot be seen."
Roleplaying Game Core Rulebook Pg.564
"Perception has a number of uses, the most
common of which is an opposed check versus an opponent’s
Stealth check to notice the opponent and avoid being
surprised. If you are successful, you notice the opponent
and can react accordingly. If you fail, your opponent can
take a variety of actions, including sneaking past you and
attacking you."
Roleplaying Game Core Rulebook Pg.102
First enter key is your friend, use it sometimes. Sorry to say it but reading that paragraph of yours was quite painful. At least you used punctuation :)
Your half-orc entering said room would not see anything (unless suceeding on a perception roll), but he knows that someone is here. Somewhere.
Or outside in the wild, the party walks, suddenly the half-orc raises his hand making everyone stop as his nose caught the scent of someone, then sniffs the air as a move action to figure out where it's coming from. (shouldn't be an unknown situation from some movies)
If your orc walks into a room with a hidden person, you should get a "you smell something" note from the DM. This doesn't mean the hidden person is in the room any more; they could have just left. Now if you start throwing down perception checks and move actions, you should be able to track the scent. This may lead you to the hidden person (who can still get a stealth vs perception, sight, surprise attempt against you, unless it lets you get within 5' of it).
However, you may also get the scent and spend a few minutes sniffing around just to find that the person simply ran through the room, or that it is a bedroom where the person's scent is prevalent.
For example (in my game at least), if you walk into a carrion worm den, your scent would simply tell you that carrion worms lived there, not that there are any present.
By the definition you quoted, "A creature with the scent ability can detect opponents by sense of smell" would let you be aware of a person's smell just like you'd be aware of them breathing loudly or standing in plain sight.