Answer:
What is in seasons, seconds, centuries, and minutes, but not in decades, years, or days?
You might accuse someone living with a stolen identity of being this.
In many hallways you would stand, if not with this in hand.
It is in every mountain, it's not in any hill, it's not in all the world, and yet it's in the mill.
What always comes into the house through the keyhole?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?