Memory:Invisible stalker

Something.. a slight ripple in the air.. an almost imperceptible shift in the scenery before you. You can feel its presence, some creature... invisible to the eye, but very real to the touch. You think you can hear it breathe, but that may be your mind playing tricks. She seems to be extremely shy; you simply can't get a proper look at her, so anxious she is to escape your attention. Just a moment ago she was hiding in your shadow, now she creeps into a crack in the wall, and now she even hides behind the air itself! There must be something beyond common stealth and dexterity behind those powers of not being seen.