The caretaker's supposition was correct.

When he returned, he was accompanied by a burly constable, who saluted us with a cheerful smile and glanced facetiously round the empty room.
"Our boys," said he, nodding towards the broken window; "they're playful lads, that they are.

You were passing when it happened, sir, I hear." "Yes," answered Thorndyke; and he gave the constable a brief account of the occurrence, which the latter listened to, notebook in hand.
"Well," said he when the narrative was concluded, "if those hooligan boys are going to take to catapults they'll make things lively all round." "You ought to run some of 'em in," said the caretaker.
"Run 'em in!" exclaimed the constable in a tone of disgust; "yes! And then the magistrate will tell 'em to be good boys and give 'em five shillings out of the poor-box to buy illustrated Testaments.

I'd Testament them, the worthless varmints!" He rammed his notebook fiercely into his pocket and stalked out of the room into the street, whither we followed.
"You'll find that bullet or stone when you sweep up the room," he said, as he turned on to his beat; "and you'd better let us have it.

Good night, sir." He strolled off towards Henry Street, while Thorndyke and I resumed our journey southward.
"Why were you so secret about that projectile ?" I asked my friend as we walked up the street.
"Partly to avoid discussion with the caretaker," he replied; "but principally because I thought it likely that a constable would pass the house and, seeing the light, come in to make inquiries." "And then ?" "Then I should have had to hand over the object to him." "And why not?
Is the object a specially interesting one ?" "It is highly interesting to me at the present moment," replied Thorndyke, with a chuckle, "because I have not examined it.