Board Thread:Theories and Speculation/@comment-26037879-20150601093706/@comment-734582-20150601111306

Mecha Naruto wrote: Natural energy is everywhere in the atmosphere, no one in Naruto world (except some few) knows what is natural energy they can just feel it passively, there is no proof that they can't feel it. To clarify: Sages can *see* natural energy.

As to sensors: I always imagined they projected a kind of chakra barrier around them that senses people (like Jiraiya's barrier against Pain). Some have a more special form, like Karin who seems to have an Ino-like sensor ability. It could be that "sensor" type is just a name for a collection of different jutsu that achieve the same effect, and your mileage varies from type to type.