I'm re-reading The Hobbit and I was wondering why there was no reference to Balin and Dwalin accompanying Thrain II on his attempt to go to Erebor. It seems to me that in The Hobbit they act as if they didn't know anything about that journey. Is it something that Tolkien added later just to the LoTR appendix?
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The mention of Thráin in The Hobbit only served one purpose: to establish how Gandalf acquired the map and the key. There was no thought-out backstory at that time. While some changes (such as how Bilbo acquired the Ring) were retconned in a later edition of The Hobbit, this detail was likely considered too inconsequential to include (even if Tolkien himself had noticed the discrepancy).
More information about how The Hobbit came to be part of the larger universe can be found in How much of the Middle Earth universe had Tolkien invented when he wrote The Hobbit?. In short, The Hobbit was unrelated until The Lord of the Rings was written, at which point both were posited as taking place in the same world as that described by (the yet unpublished) Silmarillion, but at a later time.
That said, there may be some mention of this in The History of The Hobbit, which I have not read since shortly after its publication in 2007. [Update: @ibid added a relevant passage to their answer.]
Also, there's simply no mention at all of Balin or Dwalin reacting to the story of Thraín in The Hobbit. We can simply assume that all the dwarves present are familiar with the story of Thraín setting out, and that any further exposition was possibly cut off by Thorin's curt dismissal of Gandalf's introduction.
[Gandalf speaking] "And Thrain [sic] your father went away on the twenty-first of April, a hundred years ago last Thursday, and has never been seen by you since--"
"True, true", said Thorin.
"Well, your father gave me this to give to you; [...]"
One can easily imagine Balin and Dwalin nodding along in agreement with the story they themselves were part of as Gandalf began it, until Gandalf skips to the end after some impatient prompting by Thorin.
Thrain's journey with Balin and Dwalin was created during one of the final stages of the writing of The Lord of the Rings appendices in c.1955, over two decades after those lines from The Hobbit were written.
The first version of it went as follows,
Partly by the very power of the Ring therefore Thrain after some years became restless and discontented. He could not put the thought of gold and gems out of his mind. Therefore at last when he could bear it no longer his heart turned again to Erebor and he resolved to return. He said little to Thorin of what was in his heart. But with Balin and Dwalin and a few others he arose and said farewell and departed (2841).
The Peoples of Middle-earth - "The Making of Appendix A" - page 280
In The History of the Hobbit, John Rateliff discusses these much later developments that Tolkien would make and how they affect The Hobbit. He concludes that they do not perfectly line up.
The one incongruity between this later account of Thrain’s loss in Appendix A and that given earlier in the opening chapter of The Hobbit is the fact that Thorin should have known far more about his father’s fate than seems to be the case, since two of Thrain’s companions from that earlier expedition, Balin and Dwalin, are among his own companions on this quest. Although a relatively minor point, this is still notable as one of the very few points where the two books fail to completely sync up, like Bilbo’s apology to Gloin at the Council of Elrond
The History of the Hobbit - VIII Mirkwood - iii The Theseus Theme*
