Centaur acute vision

Some kind of bug with centaur acute vision. Sometimes I can see invisible people in the wilds, sometimes I cannot. Can't figure out system behind it. A Narrow Rocky Path A narrow path winds its way along the face of the mountain. Glancing down, you suddenly realize how high up you are as you look down on a broad canopy of trees that grows more dense to the southeast. The path itself grows ever more treacherous to the east, but seems to be well defined by tiny piles of gray rock. You can follow the narrow path to the west or to the east. [Exits: east west] wilderness MOUNTAIN <100%hp 100%m 99%mv 10440tnl (20.91%) 9 PM> scan e You scan east. ***** Range 1 ***** Gliding along in the darkness, a dark-elven scout looks for something to kill. ***** Range 2 ***** A dark elven scout is sleeping here. --------- Notice there's noone here but elven scout wilderness MOUNTAIN <100%hp 100%m 99%mv 10440tnl (20.91%) 9 PM> e You begin creeping in that direction. A Narrow Rocky Path A narrow path winds its way along the face of the mountain. Glancing down, you suddenly realize how high up you are as you look down on a broad canopy of trees that grows more dense to the southeast. The path seems to be well defined by tiny piles of gray rock. To the east the path opens up to a broad and flat outcropping of rock. To the west the path seems to grow more treacherous. [Exits: east west] Gliding along in the darkness, a dark-elven scout looks for something to kill. wilderness MOUNTAIN <100%hp 100%m 99%mv 10440tnl (20.91%) 9 PM> scan e You scan east. ***** Range 1 ***** A dark elven scout is sleeping here. (Invis) A slender arial with great gray wings is here, keeping a watchful eye. ----- See how I can see invisible person scanning 1 pace? wilderness MOUNTAIN <100%hp 100%m 99%mv 10440tnl (20.91%) 9 PM> e You begin creeping in that direction. A Flat, Rocky Outcropping A broad, flat outcropping expands towards the south. A deep fire pit dug amongst the gravel gives off faint light and some warmth to the chilly night breeze. Thick slabs of rock have been set in front of the fire to prevent it from casting much light, though why anyone would wish to remain in the shadowy darkness of the night is hard to understand. Seeming to stretch out into the night, the outcropping continues to the south, and a narrow path can barely be made out to the west. [Exits: south west] A dark elven scout is sleeping here. ---- Can't see anyone here. wilderness MOUNTAIN <100%hp 100%m 99%mv 10440tnl (20.91%) 9 PM>