"Your new mark has heightened your senses. Concentrate and you can sense everything around you; even through the walls."
Farsight allows the player to see everything around them (enemy positions, and wiring), even through the walls.
Farsight however is not capable of providing vision to the interior of vents whilst the player is out of them.
Farsight could be performed while in a hiding spot, and although it may seem as if The Ninja peeks out of his hiding spot, he will not be revealed.