Disclaimer: I do not own the Lord of the Rings or any of its characters (more's the pity)
Faramir, son of Denethor of the House of Hurin, took one last look around the chambers that had been his home for the past 19 years. And the young Ranger was determined that it should be for the last time; he would not, once he had escaped, return to the Citadel by any other means than physical force or, Valar forbid, if anything should happen to Boromir and his father should die. He shouldered his pack and carefully, carefully opened the door into the corridor.
He felt a slight pang of guilt when he saw Boromir's room door hanging open, the light from his fireplace pouring into the hall. He did not truly wish to cause his older brother pain, but he simply could not bear another day of the abuse Denethor had been flinging Faramir's way for years now, and Boromir deserved more than to spend his life listening to Denethor and Faramir fighting.
Faramir ghosted down the corridor like the fox he had often been compared to and waited only for the change of the guard to make good his escape from the Citadel and then the city. No one noticed, or if they did, they did not remark upon, a lone horse and rider making their way across the Pelennor to the North.
Boromir woke the next morning with the vague sense that something was not right. He could not put his finger on what it might be, but something in the back of his mind told him that something was missing. He looked around his rooms; everything was in its proper place, nothing was glaringly ajar, in short, nothing had changed since the night before. He got up and walked down the hall toward his brother's rooms, intending to ask him whether he too felt uneasy.
Faramir was not in his rooms. This did not worry Boromir unduly; it was not unusual for Denethor's second son to rise early and Boromir fully expected to find him in the Great Library as usual. But when he got there, he found only the Steward sitting with his head bent over a book, reminding Boromir for one moment of a vulture bent over its prey.
"Father, forgive me, I did not mean to intrude. Have you seen Faramir? I had thought to speak with him - "
"No I have not. Perhaps he is on the practice grounds." There was something unsaid there, but Boromir did not ask, too preoccupied to be curious. He bowed and left, now truly uneasy. Where was his brother?
He soon ascertained that Faramir was not in any of his usual haunts, nor even in the Citadel. Guardsmen were sent out to look for the young Ranger, only to come back and confirm Boromir's worst fears. Faramir, lieutenant of the Ithilien Rangers and second son of Lord Denethor, was nowhere in the city. A further inspection of his rooms proved what Boromir had already known deep down; his brother had cut his losses and run. His spare cloak, sword, bed roll and all other necessities were gone. Boromir cursed softly and rounded on his second – in – command, Ardren.
"Go to the Tower; get a search party together."
"Shouldn't we tell Lord Dene-" Ardren caught sight of the look on Boromir's face and paled, " – thor. Right, yes, of course my lord." He bowed hastily and hurried away.
Boromir was halfway to the army barracks when Denethor caught up with him. "Where are you going with half of the Citadel Guard?" he asked in a tone that was both puzzled and irritated.
Boromir was no fool; he could guess the reason for his brother's sudden departure. He regarded his father coldly. "I am going out to try and find my brother, your son, who appears to have tired of being trod upon by his own father, my lord." Boromir made the title sound as though it left a bad taste in his mouth. Denethor appeared surprised for about two seconds before a mask of cool indifference settled on his face, hiding whatever emotion he might have felt. Then he spoke, and for the first time in his life, Boromir saw the man his brother had run from.
"Why look? He will come begging home when he lacks for food and bed, and if he does not so much the better." Boromir stiffened visibly.
"Tell me I did not hear you say that." Denethor said nothing and Boromir snapped. "You did. You honestly… you poisonous old snake! What did you say to him? What venom drove him from home in the middle of the night?" Denethor still did not speak, perhaps in shock at this sudden tirade from his beloved eldest. Boromir was fuming.
"I go now to find him, and when we return, you will beg on bended knee for that boy's forgiveness." He did not see Denethor falter, nor did he observe the mad light leaving the Steward's eyes as he turned away.
"Boromir!" Denethor started in a shocked tone, but it was too late. Boromir had already stormed out, heading straight for the Tower of Guard.