“Look at this, boys!” he cried; and then read the following: 

  “’Look over your safe very carefully.  You may discover something to
  your advantage.’”

There was no signature.

“Who sent that?” came from Sam and Tom simultaneously.

“I don’t know.  It isn’t signed.”

“’Look over your safe very carefully.  You may discover something to your advantage,’” repeated Tom.  “Say! that looks as if somebody knew something about the robbery!” he went on, excitedly.

“We have looked over the safe a dozen times,” returned Sam.  “It hasn’t furnished the slightest clew.”

“We’ll go over it again,” broke in Dick, who had already left his desk and gone to the strong-box.  He worked at the combination for a few moments, and pulled open the safe door.

“Maybe we ought to have a light here,” suggested Tom.  “It is rather dark in this corner.”

“Wait, I can fix that,” said Sam, and reaching for a droplight that hung over the desk, the youngest Rover commenced to unfasten the wire by which it was held in position.  By this means he was able to shift the light so that it hung directly over the opening of the strong-box.

“Nothing unusual about the door or the combination that I can see,” said Tom, after all had made a careful inspection.

“And the sides seem to be all right,” added Sam.  “Maybe it’s the back or the bottom.”

“If it wasn’t so heavy we might be able to swing the safe around away from the wall,” said Dick.  “But wait, hold that light closer, Tom, and I’ll see if I can find out anything from the inside.”

Dick was now on his knees and feeling around the back of the safe with his hand.  Presently he found a crack, and inserting his fingers he gave a push.  Much to his astonishment a portion of the safe back slid upward.

“Hello, I’ve found something!” he ejaculated.  “There is a hole in the back of this safe!”

“You don’t say so!” cried Sam; and he and Tom peered into the steel box.

Then Dick continued to work around with his hand, and presently was able to slide another section of the safe back upward.  He now found that he could touch a piece of board which evidently took the place of some plaster that had formed part of the office wall.

“There must be a small trap door there, leading to some place outside,” said the oldest Rover boy.  “We’ll go into the hall and have a look.”

It did not take the eager youths long to reach the hallway of the building, and once there, all three hurried to the spot where they thought the opening might be located.  Soon they came to the little closet which the janitor had once mentioned to them—­ a small place in which was located a sink, and also a number of brooms, brushes, and cleaning cloths.

The closet was dark, but Dick had brought along a box of matches, and a light was quickly made.  A corner containing some brooms and cloths was cleaned out, and the boys soon located a piece of board about eight inches square, covered with a sheet of tin painted the same color as the wall.