https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/72744-passive-perception-noticing-traps

“Passive perception is on whenever you’re conscious and aware.”

“It’s always on–that’s the baseline. Now, this brings up questions because then people were saying, ‘how it is when I make an active perception check I might get a roll that’s lower?’ Well, you aren’t. Yes, that roll is lower, but remember your passive perception is always on. So it really represents the floor of your perception.”

“If you make an active perception check, and get a number that’s lower than your passive perception, all that means is that you did a lousy job of this particular active search, but your passive perception is still active. You’re still going to notice something that blips onto your passive perception radar. Really, when you make that roll, you’re rolling to see, ‘can I get a higher number?’ If you fail to, your passive perception is still active. It is effectively creating that minimum.”

– D&D Lead Rules Designer Jeremy Crawford, D&D Podcast 4/27/2017 @ 22:14