’I seem to hear a faint knocking, but it’s that faint as you scarcely can hear it.  There! it’s stopped.’

‘He is inside,’ Shawn whispered.

‘Who’s inside?’

‘Mr. Hugo.’

‘It’s God help him, then,’ said the patrol, ’if he’s there long.  There’s no ventilation, Mr. Shawn.  We’d better telephone for Mr. Polycarp.  The other key will be in the key-safe.  I can get it.  But how do you make out, sir, that Mr. Hugo can be in there?  The vault could only be locked by Mr. Polycarp and Mr. Brown together, and surely they couldn’t both—­’

’Mr. Polycarp left his keys behind by accident.  He had gone before Mr. Hugo came down.’

‘There’s been no Mr. Polycarp here this morning,’ said the patrol a minute later.  ’I’ve looked at the signature-book.  I thought it was queer I hadn’t seen him.  And, what’s more, that isn’t Mr. Polycarp’s vault at all.  Mr. Polycarp’s vault is No. 37.  This vault has been empty for several weeks.’

‘Then you have both the keys?’ Simon demanded quickly.

’No, sir.  It’s very strange.  There’s only one key of No. 39 in the key-safe, and it’s the renter’s key.’

‘Then Mr. Brown must have the other.’

‘I expect so.  But he ought not to have.  It’s against rules,’ said the patrol.  ‘I know where he takes his lunch.  I’ll send for him.’

Simon put his ear again to the face of the door.  The faint knocking had ceased, but after a few seconds it recommenced.

‘And suppose you don’t find Mr. Brown?’ he queried, still listening.

’Then that vault can’t be opened.  But never you fear, Mr. Shawn.  I’ll have him here in three minutes.  It’s funny as he should have left anybody in there by accident—­and Mr. Hugo of all people in this blessed world....’

The patrol’s accents died away as he passed down the main corridor.

Within the next half-hour Simon, who had the rare virtue of being honest with himself, was freely admitting, in the privacy of his own mind, that the crisis had got beyond his power to grapple with it, and he had begun to fear complications more dreadful than he dared to put into words.  For the patrol had failed to find Mr. Brown.  Mr. Brown, head guardian of the Safe Deposit, had disappeared.  Nor was this all.  A renter had come to take his belongings from a safe in the third side-passage on the left, and the sub-guardian imprisoned in that passage could not open the grille between it and the main corridor.  He had his key, but the key would not turn in the glittering lock.  The renter, too impatient to wait, had departed very angrily at this excess of safety.  Then it was gradually discovered that every sub-guardian in every side-passage was similarly imprisoned.  Not a key in the entire place would turn.  The patrol rushed to the main door.  The three keys had clearly been turned while the door was opened, and the shot bolts prevented the door from closing.  This explained why the door was ajar, but it did not explain the absence of the doorkeeper, who had apparently followed in the footsteps of his chief, Mr. Brown.