is a simple cautionary expression that can be quickly & easily uttered under
one's breath without pursing of the lips (in other
words- it is possible to exclaim through the teeth with little movement of the jaw and mouth).. thus, an innocent smile may be seen maintained and unbroken on the
face of a lookout as he or she directs accomplices to halt their actions- '
cheese it' is simply a warning to desist that can be spoken by a guard without giving himself away as an accomplice or drawing attention to the forbidden act at hand.
CHEESE IT! or
cheese it! would not necessarily be used in a sentence unless in literature or in film and, then, only to better convey to the audience the reason for it's use. For instance- to say
CHEESE IT! will suffice in common usage- adding "THE COPS!" is completely unnecessary.