Here's why:
* Abstract nouns refer to concepts, ideas, qualities, or states that cannot be physically touched or seen. Examples include love, happiness, freedom, and justice.
* Adjectives describe nouns or pronouns, providing more information about them.
"Unnoticed" describes a state or condition, but it's not a concept in itself. It modifies a noun, like in the sentence "The bird flew unnoticed."
To make "unnoticed" into a noun, you could use the noun form "lack of notice" or "being unnoticed."