To add to the other answer, there are three questions at the heart:
- Target is in another dimension:
For a more Demon-sourced answer, let's look at the Process: Eliminator Cannon (Flowers of Hell, page 128.) It explicitly states "No physical range, but crossing into a different plane of existence (ie, Underworld) passes the Demon out of range."
According to CoD/WoD cosmology in general, different realms of existence cannot interact - the Demon and target must be in the same plane of reality.
- Target is supernaturally concealed:
This form ability allows a Demon to see a person in their location (or location.) So we have two stipulations:
- Demon Doug is trying to look for Nosferatu Nick. Nick is at the park, wearing his favorite band shirt, in full Obfuscate.
When Doug tries to look at Nick, he can effortlessly see the empty park. If he wants to try and actually tell what Nick is wearing, that would then be trying to pierce the Obfsucate, and then the Clash of Wills**.
- The other stipulation would be Doug wants to look at the park as a location, but Nick has used advanced Obfuscate to hide it.
In that case, Clairvoyant Sight would turn up nothing, although potentially, a Clash of Wills. Personally though, Doug should prolly get suspicious if his favorite park is suddenly "gone."
- Target is in Twilight
Twilight is different because Twilight is not a separate plane, but a unique way to exist. Since Angels interact with Twilight a lot, it's worth breaking down as a special case.
The easiest way to describe Twilight is that it's a different physical "language." For example, Doug looks into a room where there are three horses and vier Pferde. He sees 3 animals, because he can't comprehend the others. Bound Barry (thanks to his Geist, Gary) can see all 7 animals.
Assuming Doug has no way to see into Twilight, if he tried to focus on someone in Twilight, he would again see the location with "no one" there.
** Clash of wills applies when two preternatural creatures attempt conflicting powers. Eg, Mage Mary tries to see the aura of Beast Betty while Betty tries to go unnoticed. Mary can only see the aura if Betty is noticed, and Betty can't go unnoticed if Mary is studying her aura. Thus, Clash of Wills to determine who wins.
AFAIR, Demons do not have an easy way to pierce supernatural illusions (like Vampires' Obfus-"don't look at me"-cate.) However, I'm including it as an option because it would make sense.
A "see through illusions" power would likely be a custom Exploit, though - from the GM's perspective, seeing a "normal human" perfectly watch an invisible Vampire would sure seem abnormal, and surely be flagged for later review.