Thanks for all your replies guys! Really helpful! 
So say the orcs arent really looking. But their PP is higher then the stealth check of the elf. What happens then? I thought the orcs now notice the elf and can attack it. But your statement suggest that they need to make an active check again to determine if its indeed an elf instead of say a wild boar.
In the same post you say that PP is not something like a general radar for anything. You have to declare "im looking for elves". But if the orcs PP score is higher then the elf hide check, isnt it reasonable to assume the orcs have spotted the elf, because that was their spotting objective.
Situation 1: the orcs are not really looking
1. let the player make a stealth check compare vs PP of the orcs. (optionally: give orcs disadvange)
1a. the orcs notice something. Now let the orcs make an active perception roll to see if they see the elf. if they succeed they can attack, otherwise they march on.
1b. the orcs dont notice anything and march on.
Situation 2: the orcs are on the lookout for elves
1. let the player make a stealth check, and again compare vs PP of the orcs
1a. if the orcs PP is equal or greater they see the elf immediately and can attack.
1b. otherwise they march on.
Is this reasonable?

And here’s the thing, something triggering passive perception doesn’t mean the character knows what it is, just that something seems out of place, and needs a closer look (active perception) or proper investigation.
So say the orcs arent really looking. But their PP is higher then the stealth check of the elf. What happens then? I thought the orcs now notice the elf and can attack it. But your statement suggest that they need to make an active check again to determine if its indeed an elf instead of say a wild boar.
In the same post you say that PP is not something like a general radar for anything. You have to declare "im looking for elves". But if the orcs PP score is higher then the elf hide check, isnt it reasonable to assume the orcs have spotted the elf, because that was their spotting objective.
Situation 1: the orcs are not really looking
1. let the player make a stealth check compare vs PP of the orcs. (optionally: give orcs disadvange)
1a. the orcs notice something. Now let the orcs make an active perception roll to see if they see the elf. if they succeed they can attack, otherwise they march on.
1b. the orcs dont notice anything and march on.
Situation 2: the orcs are on the lookout for elves
1. let the player make a stealth check, and again compare vs PP of the orcs
1a. if the orcs PP is equal or greater they see the elf immediately and can attack.
1b. otherwise they march on.
Is this reasonable?
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