Possibly Hugh moved too soon, for it would have been much wiser had he waited until the sneak thief actually had the locker open, and disclosed his full intention.
Urged on to action by his indignation, Hugh started forward.

Thad, realizing that it was his chum's intention to do something radical, skipped off a little to the right.

He fancied that should the skulker take the alarm and try to flee, making for the open window in the rear, he was apt to turn aside and try to pass by; so his move was intended to block this little game.
It turned out to be needless, for so interested was the fellow with the flash-light in his work of inserting a key in the lock, and trying to turn it, that he did not appear to notice anything wrong until Hugh was close at his elbow.

Then, as Thad slipped around to one side to cover all lines of retreat, Hugh reached out a hand and caught hold of the fellow by the shoulder.

At the same time he exclaimed in a severe voice: "Well, what are you doing here, I want to know, trying to break into my locker ?" The other gave a tremendous start, and a low, bubbling cry, half of fright, and also of disgust, came from his lips.