Happened in EQ1 a couple of times in my guild. A guy showed up in guild manager that we didn't know. Everyone in guild was someone we knew, it was a small family guild.
So I sent a tell to the guy after he wouldn't respond in guild chat. Turned out he had no clue what I was talking about. He wasn't in the guild, he was just showing up in the tool as being in the guild.
In other words, a glitch.
I didn't know what to believe, but it happened again a few months later, and eventually SOE even released info that there was in fact a glitch like this.
SOE doesn't need to put themselves in game to listen in, every tell you send, every word you type, is going through their servers.
The fact that the toon doesn't even show up on your server list might simply imply that the game screwed up and showed a player from another server, maybe that server's zone and your server's zone were actually on the same physical server or something, who knows? I don't know enough about their server structure to even begin to speculate.