The Didact's Symbol/Glyph

Sorry if this overlaps with another thread but I just needed to have something cleared up for me. We all know that the Didact’s symbol is displayed during one of the Halo 4 trailers. I was just wondering where it came to be known or confirmed that this was indeed his symbol. I know that I had seen it before the Halo 4 trailer but for whatever reason I cannot remember where. So help me out. Where did his symbol first show up?

Halo 3 Terminals.

It’s in the 10th anniversary terminal.