This is the 13628th most frequent Russian word.
незамеченным
"Unnoticed."
The word "незамеченным" is used here as an instrumental form, indicating the state of being unnoticed by others.
Он прошёл через толпу, оставаясь незамеченным.
He walked through the crowd, remaining unnoticed.
In this sentence, "незамеченной" is used as an adjectival participle to describe the state of the mistake.
Моя ошибка осталась незамеченной в отчёте.
My mistake went unnoticed in the report.
Here, "незамеченным" is utilized to convey the objective of recognition, contrasting with being overlooked.