From Yusef, Feversham obtained particulars enough to guide him to the house where the letters were concealed in the inner wall.

But Yusef was no longer to be trusted.
Possibly Feversham's accent betrayed him.

The more likely conjecture is that Yusef took Feversham for a spy, and thought it wise to be beforehand and to confess to Mohammed-el-Kheir, the Emir, his own share in the concealment of the letters.

That, however, is a mere conjecture.
The important fact is this.

On the same night Feversham went alone to old Berber." "Alone!" said Ethne.