I’ve done the introductory SAR mission, which was pretty easy to find the missing person with the orange smoke, fine.
But now I’m doing SAR mission and I can’t find the person/smoke.
I’ve done three missions in snow covered mountain regions where orange smoke would be easy to spot. But I can’t find anything.
What triggers the smoke to appear? Do I have to be at a certain height above the person ot trigger it or do I actually have to spot the person without any smoke by myself?
Ok, I tried another one. Finally I saw the smoke appear, half way across the search area. I flew at least two times over that exact spot, but the smoke didn’t appear. At the end of that mission I got 45 % rating for time for search.
What triggered the smoke when I was on the other side of the area instead of appearing when I was actually flying over that spot?
Ya, the SAR missions probably have the only helpful voice over in game:
“Keep going… you’re doing great.”
Several times I’ve come close to calling off the search. 
Luckly through sheer persistance, I’ve always been able to eventually locate the person. I set my external view to view behind me for the exact reason you observed. The victim often doesn’t puff smoke until after you’ve flown past them. Also, not all of the victims have a Starflash you can see before they puff smoke. (But that would be expected IRL too)
Oh, and that “They have a bright orange backpack. It should be easy to spot.”
I would like to know if any real life SAR pilots were ever told something like that over the radio during their search. Because that’s a bunch of BS right there. Not even their fully erect bright orange tents are “Easy to spot from up there”
Being able to successfully land and rescue them is a different matter.
What I noticed is that the smoke is not triggering instantly, you need to fly somewhat close by to the person. First they will use flashes to get your attention, in some cases this is visible from long distance (like using a mirror or something), in some cases not at all, like in forests or obstructive mountain area. When you fly a search pattern, you can actually fly by them as till they trigger the smoke you already passed them. For me it worked to go in a specific pattern and redo the pattern, this way the smoke has already been triggered and will be visible.
Edit: I just saw that Kapustick already wrote this. I can confirm that as well.
The time to find them shouldn’t influence my rating, if the smoke coming up is totally random, right?
Does altitude matter for triggering the smoke?
The smoke isn’t random. It seems triggered by proximity.
I have found that flying at lower altitude at slower speed helps give them time to puff smoke before you leave the area ( I usually fly at around 70 kts or less at just high enough to see a larger area, but not too high to miss seeing a tent.
Canned NPC dialogue might give it away at times too. “Don’t play around. I told you only to use this frequency…” or whatever they say, usually gets said when you overfly the victim.
A trick for doing the rescue I’ve found is to land, and instead of taxiing to the victim, you exit the aircraft and run to them on foot. As soon as you get close to them, the victim runs to your plane. You still have to close-ish or it’ll trigger a “You’ve abandoned your aircraft” failure, but it might save your plane from damage due to terrain.
The trick with landing a bit further and go to them works. You need to get about 0.03-0.04NM close to the target to trigger the scene, it works on foot.
The “Don’t play around with the frequency” dialog plays when you press the button/key to indicate that you found the person, but the person is not near you.
Just keep spamming the “I spotted them” button. (Enter key) every few seconds.
When you’re in range, it will say “Found them!”
What’s really embarrassing is when you find them, then they stop blowing smoke, and you lose them and can’t find them again. I looked for a way to put a POI marker on the EFB map, but didn’t see it.
Well, from watching some FS24 SAR missions on YT it seems like a sad fact that all these missions are always exactly the same. Same search area and even same spot for the missing person. And it’s also the case that it doesn’t matter how close, how low or how slow you fly at the spot, the missing person will only activate on a scripted timer.
At least I would have expected randomization for the missing person spot. That would be the bare minimum. What’s the point of SAR if you already know where the person is going to show up?