I was playing from the PC and I saw a strange ID, it is supposed to be a maximum of 4 letters but it had more than 4 and a different font, after 3 minutes this person left the game, maybe he was banned? I do not know
From what I've heard, some folks have hacked the game to display the Bungie tag, which is how Bungie employees would've appeared back in the day when they were playing Reach. I've seen it a handful of times myself.
It's modded. Back in the OG Reach days, Bungie (The original Halo devs) developers would have a unique service tag. Back in the day, you can have 4 letters, unlike now (343 please bring back 4 letter service tags like in halo 5).