The most common and most likely answer to this crossword clue is the 11 plus 5 letter word COMBINATION LOCKS
I've seen this crossword clue in The Guardian
C
O
M
B
I
N
A
T
I
O
N
L
O
C
K
S
Buddy explains!
The straight part
We know that "might they be on the case?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case might they be on the case? = combination locks.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.