How does Nondetection protect from divination magic?
When someone casts Clairvoyance or Scrying on a place/creature other than the target and there is something/someone protected by Nondetection in the perceivable range, what does the caster perceive? Will they see a blank area? Will they see as if the protected creature/place is invisible? Will they know that there is an area they miss?
The exact phrasing of the spell is:
The target can't be targeted by any divination magic or perceived through magical scrying sensors.
Thus, you cannot be perceived through magical Scrying sensors. This means that even if you target a location with the Scrying spell, you are not perceived--to you, the target is both silent and invisible. You might be able to ascertain that someone is there by how the environment reacts (footprints, etc.) but you cannot see them.
It's a little less clear whether you're protected from a Clairvoyance sensor, since the text of Nondetection specifically says "scrying sensor" and not "clairvoyance sensor". However, the PHB uses the lowercase "scrying," so it's likely referring to sensors in general, and therefore you'd be invisible to a clairvoyance sensor also.
This means that the casting of clairvoyance and scrying will fail if they attempt to target the object, location, or creature affected by nondetection. However whether a spell slot is expended is debatable (see this question).
Xanathar's Guide to Everything details this as a potential ruling:
If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.
