The Secret
The Prophet gave Hudhayfah a list — verbal or written, accounts differ — of the names of every person in Medina who had outwardly accepted Islam but was a hypocrite (munafiq) in the Quranic technical sense: those whose allegiance was calculated, who would harm the community if given the opportunity.
This information was not made public: the Prophet’s approach to the hypocrites was to treat them as Muslims in law while being aware of their interior state. Making their names public would have torn the nascent community apart.
Hudhayfah carried this secret for the rest of his life.
The Practical Consequence
The practical consequence of Hudhayfah’s knowledge: when a prominent person died, the Caliph Umar would wait to see whether Hudhayfah attended the funeral prayer. If Hudhayfah prayed over the deceased, Umar knew the person was a sincere Muslim. If Hudhayfah abstained, Umar knew otherwise — and would also not pray.
This made Hudhayfah an unofficial authentication authority for sincerity in the community — a role he carried with discretion.
The Night Reconnaissance at the Trench
During the Battle of the Trench (5 AH), when both sides were locked in a cold stalemate and the Muslim commander needed intelligence about the Quraysh camp, the Prophet sent Hudhayfah alone at night to infiltrate the enemy position, count their forces, and assess morale. Hudhayfah reported back that the enemy’s morale was collapsing — information that contributed to the Prophet’s decision to send a diplomatic agent to break the Quraysh-Qurayza alliance.
See also: Seerah Khabbab Ibn Al Aratt, Seerah Usama Ibn Zayd, Seerah Sad Ibn Muadh, Seerah Abu Dharr Al Ghifari, Ilm Al Firaq