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.
