There was no one in sight and he stooped down and examined the stairs carefully. Then he straightened up and rubbed his chin as a sudden gleam of intelligence passed through his brain. "So," he muttered, "this is why Santiago dwells in a house that is directly back of the banker's. That was his box and he is the strange man who made the million-dollar deposit in Don Esteban's bank." Then he stopped and pondered. "But who stole the box? Not Santiago, for he has not left his room for days. If it was Strong, he must have entered the bank from some direction other than this. I don't understand, but I'm going to make some further investigation." He reentered his room, buckled on his automatic and took from his suit case--which, by the way, he had located at the railway station along with that of his companions after the occupation of the city by the marines--his electric torch. Then he went out and descended the stairs, which he discovered were twenty-four in number. Reaching the bottom he found himself upon a landing some six or eight feet square, from the opposite side of which another flight of stairs ascended. "I reckon I better see where they lead to," was his comment as he slowly began their ascent. After going up eighteen stairs his head touched the floor above. He counted the remaining steps by the light of his torch and found that there were six more. This would make the floor over his head on an exact level with the floor of Santiago's house. "It's just as I expected," he muttered. "If I can get through this floor, I shall find myself in Don Esteban's residence--somewhere. But just where? That's the question." He pushed upon the stone above his head, but it refused to move. Then he held up his torch and examined the ceiling carefully. Whatever the method by which the stones could be moved, it was carefully concealed. Much chagrined, Billie at length decided to retrace his steps and await the coming of his companions. He reached the landing and crossed over to the steps by which he had descended. Glancing up, he uttered an exclamation of surprise, for there at the opening and peering down the stairs was Ambrosio, the ape. With a cry of recognition the simian started to descend the stairs, but at a noise from above he hesitated and then sprang back and out of sight. An instant later the opening was closed and Billie was left in the darkness, except for the light of his torch. CHAPTER XXIX. THE MYSTERY DEEPENS. While Billie was making his discoveries Donald and Adrian had also been busy. When they left the house, at Donald's suggestion, they went first to the bank and told Don Esteban about the disappearance of the box and what the maid had seen. He was much disappointed until it was explained to him that they also had another clue. "We shall spend our time in the slums," said Donald, "while our chum keeps watch at the house. We feel sure that between us we shall solve the mystery."