Melchizedek the King of Salem. The hint that he is the King of Peace alluded that he was the King of Jerusalem/Yerushalayim (City of Peace). Yes, Melchizedek is "untraceable" but so are many others are not as well...seemingly. It is known that Melchizedek was another name for Shem (The 1 righteous son of Noah). Many people have multiple names in the Bible, Yitro/Jethro has 7! Jesus has tons and God has 72, so this is not uncommon.
Hebrews is, in short, trying to convey that Shem/Melchizedek was the first righteous King and Priest (very rare, so rare that Jesus is the only other) and that just as Abraham who carried forth the promise of God by being blessed by Shem, so too Jesus is our King and Priest who blesses us too (this also is one of many proofs of why we need to give 10% tithes as believers since Abraham too did the same thing to Melchizedek. Hebrews is also using the fact that the Torah does not 'verbatim' spell out Shem's identity but with Hebrew, it is easy to see. But this hidden point is correct, it is to use it that Jesus was also hidden in the text of the Torah/Tanakh and now it has finally come out.
Just a final note, the book is called "Hebrews" for a reason. This point was addressed to the Jews who would understand this much better. Very few to no Gentile in the "then" synagogues would have been taught such deep teaching, nor was it necessary for their understanding. The book of Acts shows that every time the disciples dealt with Jews, they absolutely had to stick to Torah and Tanakh because simply they are very learned in the scriptures. That also is a clue to how we also need to approach Jews.
I hope this helps! I enjoy all your discussions!