'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.