Shinobi Senses, Gaara Hiden

A person wasn’t able to keep watch 360 degrees in every direction, but he’d still be able to tell if, say, a friend was approaching from outside his field of vision, or if a pet cat was playing at his own feet.
That was because a person’s consciousness was able to reach out to their surroundings and ‘see’ what was ‘unseen’.
Shinobi forged and improved that consciousness, using even their intuition, until they were roughly able to sense all their surroundings with all their six senses.