I think some additional modifiers should be added to the Perception DC table (Sight), as follows:
Sight Modifier - DC Modifier
Concealment/Cover (Bushes, Tables or other obstacles) +4
Shadowy Illumination (for torchlight & candlelight) +2/range increment
Invisible creature +20
I think the following should be added to the Perception (Smell) description.
"You cannot use smell to detect something with a DC greater than 10 unless you have the scent ability"
And smell DC's based on race? C'mon...give us a break. How about instead:
Smell - Perception DC Modifier
Creature with Musk/Nausea attack (Skunks, Ghasts, Troglodytes) - DC -10
Filthy/Unkempt creature - DC -5
Sweating/Exerting creature - DC -2
Freshly clean/bathed creature - DC +2
"Scrubbed"(*) creature - DC +5
Pass without Trace ability - DC +10
* Creature has specifically attempted to remove all traces of a scent through washing or other means.