I agree with Li Shenron on the matter of passive Perception as a "floor." So far as I can tell, the rules only support this being the case in combat and detecting danger (because per the rules most creatures are alert to danger). Or arguably if a character is engaging in a specific task over time (e.g. searching for secret doors while traveling the dungeon) and then opts to be more reasonably specific in a particular instance of searching for secret doors that might also call for an ability check. There is nothing in the rules, however, that suggest you're always alert to danger outside of combat or always searching for secret doors (for example) such that passive Perception would always be on. It's situational.
I could probably have done a better job wording that, so if it's not clear, please let me know.
Nope, you both are quite clear. But to my mind, the rules don't really say either way how or when PP should apply.
Like I said... if you as a DM want to go through the effort of deciding on a case-by-case basis "this can be found via PP, this other thing cannot"... go nuts. To me, though, that's just an extra unnecessary step.
How I determine whether something can be found by PP? The DC tells me. If the DC is above the PC's PP, then they can't find it via PP. If it's below, then they can.
After all... if someone makes a DEX (Stealth) check and rolls a 7... why would I ever decide "Well, you know I don't think I'm going to let their PP find him, even though with that 7 it basically means his butt is sticking out. I'll wait for them to say they are going to look for him first and make them roll for it." The monster rolls way below their PP but I still make them roll Perception checks because who knows, they might roll horribly and still not see him? No thanks. To me, that's just silly. But every other DM can do what they wish.