The point is, and this undergirds the POV of people like myself, there are no 'intruders' to know or not know things! There's only a GM who is empowered to say something. D&D has always been rather poor at describing what the thought process here should be (there is no established game process).
@Robert Conley,
@Bedrockgames, et al have adequately described what I would consider a pretty mainstream position on this,
@bloodtide expounded a different one, etc. As a player I don't actually know what the risk is of some clever enemies getting around the Alarm. I have to assess THE STATE OF THE GM'S MIND for that. Is he BRG, or Bloodtide? Probably somewhere closer to BRG, but my focus is not on the fiction.
Unless you're playing with someone for the first time, there's a good chance you would already know the state of the GM's mind. You likely know if the GM is the type who would try to counter a player action (casting
alarm) via metagame knowledge (saying the intruders know you cast the spell and are trying to wait until its over, without having first done Perception checks to see if the PCs suspect that there
is an intruder).
I agree that D&D doesn't really spell things out directly in the sense that there is no single checklist telling GMs that they should (by RAW) be making a bunch of rolls to see if the intruders (a) know you cast a spell, (b) know what spell it is, when you cast it, and what the duration is, (c) manage to not be detected by anyone in the camp who is awake, aware, or otherwise capable of detecting intruders, and (d) chooses to bypass the spell in order to attack in melee range. Maybe D&D's devs didn't think of it; maybe they thought it was clear by reading the text.
And as I said, I don't know from Torchbearer. I don't know what's involved in a camping roll or what it means to have +1d to avert disaster. I don't even know what it means that the bell in Aetherial Premonition rings in "the event of trouble."
It almost sounds like a roundabout way of rolling for random encounters. Instead of saying "roll three times during the night, there's an encounter if you roll 15 or above" like you may find in a D&D game, it's more like "there's an encounter if the players fail a camping roll."
If that's the case, then it sounds like they're the same thing, it just shuffles the responsibility of determining the likelihood of an encounter occurring to the player.
I honestly don't know which one is better, mechanically. For D&D, it assumes you're rolling for random encounters. Not everyone uses them, which means a nighttime encounter is either planned or a spur-of-the-moment decision based on hopefully logical events. But for Torchbearer, if a failed camping roll means disaster, then there might be less of an impact because the players know something is coming, because
they're using metagame knowledge to know that a bad roll = encounter. (If that's actually what happens with a bad roll.)