2 Thessalonians was written in AD 54. Hebrews was written around AD 67. 13 years later.
Perhaps because of its intended audience, not a church, but to the Jewish believers, and because of it's emphasis upon the Divine authorship of the scriptures, Paul did not see fit to give his usual salutation.
I like the preface in the RcV:
Author: apparently the apostle Paul, who alone was qualified to write the deep, high, and rich content of the book. The mention of Timothy and the tone of the fellowship also point to Paul as the author.
The footnote on vs 1:1 reads:
The emphasis of this book is that God, not man has spoken. Therefore, it does not identify its writer, nor in any of its quotations from the Old Testament does it mention the speaker's name. According to the concept of the book, the entire Scripture is the speaking of God. Hence, in referring to the Old Testament, this book always says that it is the Holy Spirit's speaking (3:7; 9:8; 10:15-17)
This I believe gives support to those who suggest, "the author is God" and leave it at that.
Graftedbranch