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This question came up while answering this question. While I made the assumption that Nondetection does not protect against an Alarm in my answer, I quickly realised that the answer probably wasn't as obvious as I thought at first glance.

The spell Nondetection states the following :

For the duration, you hide a target that you touch from divination magic. The target can be a willing creature or a place or an object no larger than 10 feet in any dimension. The target can’t be targeted by any divination magic or perceived through magical scrying sensors.

Since Alarm is an abjuration spell, it does not qualify for the first condition. However, I could not find any definition in the rules for what a "magical scrying sensor" is. Looking at how the Alarm spell works, it could be defined as a magical sensor which triggers an effect when tripped.

As far as I've seen, the term "scrying" is used for magical methods of seeing something, usually at long range or for things you wouldn't see normally, such as visions of the past or the future. This single word makes up most of the issue in defining what is or isn't concerned by Nondetection.

Can Alarm be considered as a "magical scrying sensor", in which case it could be trumped by Nondetection? In general, is there a way to tell precisely whether something qualifies as a "magical scrying sensor", or is it up to the DM's judgment?

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Alarm does not create a magical scrying sensor.

There are three spells which create magical scrying sensors, and we know they create sensors because they say they create sensors: they are scrying, clairvoyance, and soul cage (quoted in that order):

On a failed save, the spell creates an invisible sensor within 10 feet of the target. You can see and hear through the sensor as if you were there. The sensor moves with the target, remaining within 10 feet of it for the duration. A creature that can see invisible objects sees the sensor as a luminous orb about the size of your fist.

You create an invisible sensor within range in a location familiar to you (a place you have visited or seen before) or in an obvious location that is unfamiliar to you (such as behind a door, around a corner, or in a grove of trees). The sensor remains in place for the duration, and it can't be attacked or otherwise interacted with.

Eyes of the Dead. You can use an action to name a place the humanoid saw in life, which creates an invisible sensor somewhere in that place if it is on the plane of existence you’re currently on. The sensor remains for as long as you concentrate, up to 10 minutes (as if you were concentrating on a spell). You receive visual and auditory information from the sensor as if you were in its space using your senses.

So how do you know a spell creates a magical scrying sensor? It’s got the word sensor in it.

Since alarm does not create a magical scrying sensor, and it is not a divination spell, it does not interact with nondetection at all.

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    \$\begingroup\$ The three spells cited all contain the word 'sensor', but none of them say they make scrying sensors, which is explicitly what notdetection protects against. Thus you seem to be arguing that we can take 'scrying' as implied from its standard meaning in English but that 'sensor' is a specific game key word and not simply 'something that senses'. This answer would be improved by explaining the justification for interpreting one word as a game term but the other not. \$\endgroup\$
    – Kirt
    Nov 7, 2022 at 18:03
  • \$\begingroup\$ Honestly, I think that last line should be first as it is the most important part of the answer... then go into detail about the sensors. Schools of magic do matter no matter how much they have tried to throw them out. +1 \$\endgroup\$
    – Slagmoth
    Nov 7, 2022 at 21:03
  • \$\begingroup\$ I second the first part of @Kirt's comment. Going by all other answers regarding 5e, the rules have no superfluous words. A "Sensor" is more generic than "scrying sensor". A reader might interpret the answer as stating that nondetection works against all sensors, which might not be the designer's intention. It is already too powerful as it is. No need to make it even more (powerful). \$\endgroup\$ Nov 8, 2022 at 17:16
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No; Alarm is (probably) not scrying

You could take the approach that only spells which explicitly say they create sensors do so, and this is comforting in that you know that these spells do create magic sensors. However, this is unnecessarily restrictive and fails upon inspection, in that it eliminates other spells and items that almost certainly make scrying sensors. For example, the divination spell arcane eye creates

an invisible, magical eye within range that hovers in the air for the duration [and through which you] mentally receive visual information

Declaring that this eye, which magically relays its sensations to you, is not a scrying sensor simply because the spell description does not explicitly say so turns what should be an immersive 'this is the way magic works' experience into an exercise in pedantry. That someone with nondetection would be protected from clairvoyance but not arcane eye does not make sense to me.

Similarly, the Hag Eye item made by a coven of Night Hags permits the hags to see through it anywhere on the same plane of existence. To rule that nondetection would not protect one from it, that it is not a scrying sensor simply because it does not contain the word 'sensor' in its description, does not make sense to me.

However, the rules do not define what a scrying sensor is. Thus, your DM (or you, if you are the DM), would have to develop a working definition of a 'scrying sensor' based on accepted English meanings of the term - which is not easy, considering the loose definition of scrying in English.

At first blush, I would rule that 'scrying' within the game refers to magic that extends your own natural senses, that gives you a direct, sensory experience of something that you otherwise would not be able to have. There are lots of spells which must somehow sense their environment in order to function as they have been programmed to, for example magic mouth, glyph of warding, and snare. But, once cast, they function autonomously without allowing you to directly feel what they are sensing.

This autonomous functioning seems to be the nature of alarm; while its abjuration magic senses the environment and reacts to creatures breaking the boundaries of its warded area, and can even alert you to the fact that such has happened, at no point do you directly sense the intrusion. It is the spell itself that "alerts you", rather than you actually feeling the intrusion as a sensation.

Similarly, a person on whom nondetection had been cast could walk through a crowd and be noted by every person they passed. Any one of those observers could report their presence to you; a confident or construct could alert you to their presence, but due to the nondetection, you would not be able to see them through a scrying sensor you had placed at the location. The nondetection spell specifically defeats magically transmitted sensations, not untransmitted sensations - even when they are collected by magical means or observers.

An interesting test case would be if you had a familiar observing a person upon whom nondetection had been cast. The familiar could use its animal senses to observe the target, and could report their presence to you telepathically. But according to the working definition of 'scrying sensor' suggested here, you would not be able to use your ability to see the target through the familiar's eyes; that is, you would not be able to use your familiar as a scrying sensor.

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    \$\begingroup\$ In plain English eye is a sensor. Thus, I find this answer based on false premise. \$\endgroup\$
    – Mołot
    Nov 7, 2022 at 17:18
  • \$\begingroup\$ @Mołot Yes, a mundane eye is a sensor in plain English. But it is not a scrying sensor, which is what nondetection confounds. \$\endgroup\$
    – Kirt
    Nov 7, 2022 at 17:27
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    \$\begingroup\$ @Kirt Less specifically Nondetection protects against Divination not just scrying sensors. \$\endgroup\$
    – Slagmoth
    Nov 7, 2022 at 21:06
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    \$\begingroup\$ @Slagmoth Nondetection protects against being the target of a divination spell. Arcane eye is a divination spell, but you send it out into the world to look at things - unlike scrying, you don't have to declare the target upon casting. Thus nondetection would not protect you against arcane eye unless the eye itself is ruled to be a scrying sensor - which I think it should, but the competing answer does not. \$\endgroup\$
    – Kirt
    Nov 8, 2022 at 1:24
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    \$\begingroup\$ @Kirt There has been an effort to make schools less important since the beginning of 5E imho. I agree that Arcane Eye is an outlier though. But I also rule that See Invisibility won't detect someone with Invisibility and Nondetection on them. Although in that case they aren't the target either... so I will have to pontificate on that a bit again. Although for a 3rd level spell with a component cost I don't see it as overpowered to rule that way. \$\endgroup\$
    – Slagmoth
    Nov 9, 2022 at 5:44

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