The most common and most likely answer to this crossword clue is the 4 plus 8 letter word SIDE ENTRANCE
I've seen this crossword clue in Financial Times
S
I
D
E
E
N
T
R
A
N
C
E
Buddy explains!
The straight part
We know that "where might they arrive unnoticed?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where might they arrive unnoticed? = side entrance.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.