Yes. Passive perception "supersedes" active perception by acting as a floor.
The party scout was correct. Passive perception does not "turn off" when you are actively searching. It only stops if you are unconscious. You always notice anything that hasn't beat your passive perception score — even if you aren't actively searching.
You can also take actions to actively search, giving you an opportunity to sense things you haven't already noticed.
For example, if your passive Wisdom (Perception) score is 15, and a monster is lurking with a Dexterity (Stealth) roll of 14, you notice them without rolling. If the monster got a 15 or higher, though, they've beaten your passive senses. Let's say that the monster has a modifier of +3 and rolled 16 on the die, for a result of 19. In this case, on your turn, you can take actions to actively search, for which the DM can call for Wisdom (Perception) rolls. If you exceed the monster's total, you've noticed it. If you get the same total or less (19, here) the situation remains the same and the monster is undetected. For that first monster — the one with a 14 total Dexterity (Stealth) check — the situation also remains the same, which is that you've already noticed them. They aren't somehow "de-noticed".
Jeremy Crawford explains in the Sage Advice section of this podcast, starting at about 15:09.
JC (at 22:16): Now, going back to passive perception... this is, as its name implies,
passive. And, it's considered to be "always on", unless you're under the effect of a condition, like the unconcious condition that says
you're not aware of your surroundings. That really... the practical
effect of that is that basically your passive perception is shut
off. Passive perception is on basically whenever you are conscious and
JC (at 23:09): Because it's passive, the player does not get to say they use it. This is a... this is something that people...
Interviewer: (Laughs) I'm using my passive perception right now!
JC: Yeah, no. It's always on. That's the baseline. Now, this brings up questions, because then people are saying that, well, how
is it that 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 aways on. So it really
represents the floor of your perception.
Interviewer: Right. That's an important distinction, though.
JC: Yes. So if you make an active perception check and you 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 really rolling to see "can I get a
higher number?" If you fail to, well, again, your passive perception score is still active. It is effectively creating that minimum.
Interviewer: The minimum. Yeah, I don't know if that's necessarily clear to a lot of dungeon masters out there, because they will be
like, well, the opposed nature of this roll means that you were
just really bad at looking, and even though the person who is
sneaking up on you only got like a five, they're able to do so.
JC: Now, many of these sorts of situations would be erased if DMs just simply remembered to use the passive perception in the
first place. Because honestly, if something's noticable by a
person's passive perception score, they should already have noticed
it. So really, the active search is trying to find something
that you haven't already noticed,
and your passive perception score represents what you have already noticed.
(Bold added to highlight the key points; italics intended to represent emphasis in the speech.)