So I moved close and listened.

It seemed that some of the plebes had been running the guard nights.

Lieutenant Denton asked the fellow Prescott, who is a cadet captain, to keep a watch and stop plebes before they had a chance to get on the other side of the guard line.
"Well, I knew the point at which plebes were in the habit of getting past the guard line, and so did Prescott, I guess.

So, a little after taps, I slipped outside the guard near where I judged Prescott would be watching.

Then, after I had heard him speak with the cadet sentry I presently stooped low in the bushes and lit a cigar.
Then I stood up straight and the glowing end of the cigar showed from where Prescott stood.