The Eye of the Lost is a passive Mental Skill that works automatically with The Passage. It allows Scarlett to see the ghosts of the dead, spirits which have remained in this world. It lasts as long as The Passage, and has no additional cost in Mental Energy beyond what's required to invoke The Passage.
The most common use of this skill is to allow Scarlett to view the ghosts of Don and Nesto as they show her how to pick locks. For that purpose, The Eye of the Lost is invoked automatically (at no cost of Mental Energy) any time Scarlett attempts to enter a locked door or search a locked chest.
Once Scarlett has also learned The Whisper, she will be able to speak with the ghosts she sees. The presence of such restless spirits in the mortal world is usually signaled by a Skeleton lying on the ground.
The Eye of the Lost is a granted skill, bestowed on Scarlett by Benedict's ghost at the Gateway Idol in the Old Temple of the Outer City. Learning it doesn't require use of any Skill Points. It is the initial skill of the Second Twilight Circle, and learning it opens the rest of the second circle for normal learning through Necromancy Teachers and Skill Points.