By your logic, actual opposed checks never happen.The Grackle said:It also says under Passive Checks (page 179):
"When you’re not actively using a skill, you’re assumed to be taking 10 for any opposed checks using that skill. Passive checks are most commonly used for Perception checks and Insight checks..."
So you are making an opposed check, but it assumes you're taking 10 for your roll (effectively setting a DC of skill+10) if you aren't actively looking.
PHB definition of opposed check says both characters roll.
PHB definition of stealth says the Stealth check is an opposed check.
If actually rolling a Perception check takes a standard action, you can only do that on your turn.
Using Stealth is in most cases also only possible on your turn.
So under what circumstances does an opposed Stealth check - where the stealther initiates by making a Stealth check and the other creatures present roll Perception checks - actually happen? If it's always against passive Perception, then the skill description would have just said that. Passive checks make sense out of combat or when creatures are not aware of your presence, but when you just shot a guy with a crossbow, I think his interest in your location is no longer quite so lackadaisical.