If Denethor hadn't committed suicide

[Formendacil]

One thing that is worth noting is that as late at the Fell Winter in 2911-12, a mere 19 years before Aragorn's birth, there continued to be a settlement at Tharbad. Unlike the general population of the surviving Arnorian Rangers, this settlement on the Greenway to Bree would have been more publicly known, and any traders between there and Rohan and Gondor would have passed that way.

Given that Tharbad was almost certainly a Númenorean settlement, or at least mixed including those of Númenorean blood--possibly also those of Bree/Dunlendish ethnicity, it isn't that strange from a Rohirric or Gondorian perspective to encounter Thorongil as a Númenorean stranger out of the North. Possibly, they might think, he would be ignorant of his ancestral origins, and unlikely to be as "pure blooded" as a Gondorian (remember that business with Eldacar? He looked and acted like a pure Númenorean, but that wasn't good enough for Gondor--he was halfblood and they knew it)--but hardly all that odd. A rarity, to be sure, but not unexpected.

I'm coming to the conclusion that it would not be strange at all from the Gondorian perspective to learn there were some descendants with Númenorean blood in the North. This was not Aragorn's secret. The secret, rather, was that the Line of Isildur persisted, and that the legal structure of Arnor had not ended in the aftermath of Arvedui, but persisted in the paramilitary Rangers and their unknown guard on Bree and the Shire (and their own settlements in the Angle, and, earlier, Tharbad). Aragorn's secret was not that he was Arnorian, but that there was still an Arnor and that he was its chieftain.

[Erendis]

I have completely forgotten Tharbant!Thanks for reminding me,Formedacil!
When I will write a fanfiction on Thorongil,I will enclude that theory too!