The most common and most likely answer to this crossword clue is the 9 letter word UNSPOTTED
I've seen this crossword clue in Financial Times
U
N
S
P
O
T
T
E
D
More clues leading to the result UNSPOTTED
Buddy explains!

The straight part
We know that "Not noticed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not noticed = unspotted.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does UNSPOTTED mean?
UNSPOTTED adjective- Not having spots.
- Unseen.
- Without stains or blots; sinless.