1. Let's say you have a party walking down a corridor with a corner at the end. Ten feet from the corner is a creature with total cover from the party (because of the wall). The creature is deaf, and automatically fails its perception check to hear the party. The creature is not stealthing, not moving, and not talking. What perception check, if any, would the party get, and at what point would they make it?
If any of your PCs can succeed by taking 10 against a DC of 40 + distance penalties, you could allow them to hear the creature breathing or shuffling its feet at the point where they make the check. Otherwise, the first PC sees it automatically when she reaches the corner.
2. When it comes to perception DC modifiers (+1/10 ft, etc.), are the modifiers applied in a straight line from the perceiver to the source, or does it take the path of least resistance, to modifiers that are most beneficial to the one making the check. For example, in the corridor scenario, assuming the party gets to make a perception check, would the DC be determined by drawing a straight line through the corner (incurring the penalties from going through a wall,or by going around the corner, avoiding the wall?
Whichever gives the lowest DC.