Sentence:  "Then, if they rise, shout the alarm at the top of your voice ... but be careful not to rouse the camp by a <WSD>false alarm</WSD>, for if you do, instead of gaining credit we shall become the jest of the whole force." 
Meaning of the word marked with <WSD> tags:  A warning sound (such as the call of sentry, the ringing of a bell, or the shriek of a siren) which turns out to have been erroneous.