[VIVIT]

On GURPS Powers pp. 163−164, in the section Detecting Abilitie, under the Obvious Effects heading, it is made clear that the No Signature enhancement hides an advantage from mundane senses.

If [an] ability isn’t based on a ... trait with explicitly invisible effects, isn’t totally passive, and isn’t enhanced with No Signature (p. B106), then the buyer must describe a set of effects that are obvious to one or more ordinary human senses ...

It also describes special senses that can detect the use of a power more directly. The text explicitly mentions No Signature again, and implies that special senses are not exempt from it.

In the case of a power, the GM should also specify one or more “special” senses that can detect all of the power’s active abilities by sensing its source or focus ... Special senses work even when normal senses aren’t available, unless the ability has No Signature.

In the Subtle Effects section after that, the text gives rules for abilities without an obvious signatures, explicitly including those with the No Signature enhancement:

Advantages with invisible effects, such as Clairsentience and Telekinesis, are genuinely undetectable to normal senses ... The same goes for abilities with the No Signature enhancement.

But then the text appears to contradict itself by describing some sort of signature that this type of ability does leave behind and is detectible by special senses, despite just a few paragraphs ago saying that special senses aren't exempt from No Signature:

In most game worlds, such abilities still leave a “psychic impression,” “supernatural residue,” or “signature” that those with special senses can track down. Psychometry is the best tool for the job, but specific forms of Detect can also work.

So, in summary:

1. Some abilities have signatures visible to the mundane senses; those with No Signature do not.
2. Certain "special senses" can detect the use of a power even when that use is invisible to normal senses, unless the ability has No Signature.
3. Abilities with No Signature do, in fact, leave a signature, which is detectible by special senses, despite the fact that things that are invisible to normal senses can be found by special ones only if they do not have no No Signature.

Am I missing something? Am I misreading something? How is this actually intended to work?

And, as a secondary question, how does this interact with power modifiers? The fact that an advantage is specially detectible at all normally comes from the power modifier, doesn't it? Wouldn't that mean you would need to put a limitation on the No Signature enhancement—or, alternatively, on the PM for that ability?

Is there something that Powers expects me to know here that I don't?