Will Nondetection prevent an Alarm spell from triggering

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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?

Best Answer

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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