Invisible.

Invisible objects slip unseen through crowds,
Never noticed, never given though to.
Visuals deceive our overworked eyes,
Ignorant of anything unexpected or odd.
Sight is the most useful and overworked sense,
Instantly rejecting alternate realities.
Blinking through the world, unaffected by change,
Like an unseen malaise of death,
Ever creeping, never sleeping.
...The most obvious things are the hardest to see.