Yes, it's from
The Lord of the Rings, Appendix A, ANNALS OF THE KINGS AND RULERS, I Númenorean Kings, (iv) GONDOR AND THE HEIRS OF ANÁRION,
The Stewards:
Quote:
'Thorongil often counselled Ecthelion that the strength of the rebels in Umbar was a great peril to Gondor, and a threat to the fiefs of the south that would prove deadlly, if Sauron moved to open war. At last he got leave of the Steward and gathered a small fleet, and he came to Umbar unlooked for by night, and there burned a great part of the ships of the Corsairs. He himself overthrew the Captain of the Haven in battle upon the quays, and then he withdrew his fleet with small loss. But when they came back to Pelargir, to men's grief and wonder, he would not return to Minas Tirith, where great honour awaited him.
'He sent a message of farewell to Ecthelion, saying: "Other tasks now call me, lord, and much time and many perils must pass, ere I come again to Gondor, if that be my fate." Though none could guess what those tasks might be, nor what summons he had received, it was known whither he went. For he took boat and crossed over Anduin, and there he said farewell to his companions and went on alone; and when he was last seen his face was towards the Mountains of Shadow.
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But from the above quote, you can see that he did not sneak secretly away from Gondor.