Basically, some linguistics guy was researching something else in a Byzantine codex, and started realizing that these Greek sermons in the book sounded awfully like a surviving Latin version of an Origen thing that didn’t survive. So they did some analysis, and it turned out all the Greek sermons in the book were Origen originals, many totally lost before!
It’s not what we don’t have; it’s what hasn’t been properly catalogued. It also goes to show that you should read outside your field.
If you’ve never read Origen, he was basically a good guy and a big Bible scholar, who was also a very speculative theologian. Some of his “students” (ie, the kind who read his books, not the kind who studied with him) later on went all heresy and claimed they were teaching his ideas; but he himself was loyal to whatever the Church taught. So he’s kind of a fun read, if you are ready to roll your eyes when he goes too far into speculation. St. Gregory Thaumaturgus did study under him (and was converted to Christianity by him); we have the text of a thank-you speech by him to Origen that details Origen’s teaching methods.