When he came down stairs in the morning he was met by the information that the ha'nt had robbed the safe.

You can see what instantly jumped into his mind--some way, somehow, Jeff had taken those bonds--and yet figure on it as he might, he could not see how it was possible.

The robbery seemed to have occurred while he was away.

Could Jeff merely have pretended to leave?
Might he have slipped off the train again and come back?
Those are the questions that were bothering Radnor.
He was honest in saying that he could not imagine how the bonds had been stolen, and yet he was also honest in not wanting to know the truth." "He might have confided in me," I said.
"It would have been a good deal better if he had.

But in order to understand Rad's point of view, you must take into account Jeff's character.