Is it possible to see someone's alt characters?

I used to play this game a lot years ago and recently I decided to make a fresh start with a new character on a different server.

I made a small fc with my friend and I also transferred here my old character. I checked my friendlist and deleted some people I couldn't remember and haven't logged on it again.

I was doing msq on my new character when I got a tell from someone who was on my old character's friendlist and previous home world. They said "welcome back". My characters' names are not even similar. How is it possible? Is it a new addon I'm not familiar with? How can I play incognito? It's very creepy...